Gratitude is expressed to the following people for references or other invaluable assistance: F. Jack Eastman, Loïc Jeanson, Bill Kast, Sara Schechner, Richard E. Schmidt, Patrick Seitzer, Phil Shoemaker, the late Craig B. Waff, and the two anonymous reviewers.

1During the first half of the 19th century, the United States expanded westward across the North American continent so rapidly that by 1848 the nation had acquired territory approximating its current transcontinental borders, its area exceeding that of Australia. During the decade of the 1840s a scientific community evolved along with it, most notably in astronomy (Lankford, 1997, p. 6). Although the federal government expressly forbade the building of a national observatory, by the early 1840s, it had become clear to those members of Congress who were to oversee the publication of Lieutenant Charles Wilkes’s four-year expedition that they were unfamiliar with the geology, botany, and especially the geographical results obtained during the expedition — that what they needed were “men of science” familiar with those fields. In 1842, President John Tyler signed a bill authorizing the establishment of a Depôt of Charts and Instruments, which eventually was renamed the National Observatory and ultimately the U.S. Naval Observatory (Dick, 2003, p. 27-59, 551-553).

2The next year (1843), the U.S. Congress appointed Alexander Dallas Bache Superintendent of the United States Coast Survey, bringing to bear one of the most capable scientists in the country on the problems of mapping the coastline and publishing accurate charts that employed the most advanced scientific techniques then available. Bache exemplified the “geophysical tradition” in the new country — one that utilized the most advanced scientific instruments and techniques on the problem of longitude, often pushing the boundaries of astronomy (Dupree, 1986, p. 61, 100-105; Reingold, 1991, p. 127-155; Slotten, Numbers & Livingstone, 2020, p. 641-711). He also called upon the astronomical observatories that were springing up around the new nation (Musto, 1968) to help determining latitudes and longitudes, often lending them instruments for the purpose. The astronomers were willing collaborators because much of their work then depended on exact knowledge of their own geographical positions. The observatories in turn then served as reference locations for mapping, which eventually included the establishment of state or federal boundaries, the surveying of potential routes for canals and railroads, and the charting of natural resources for mining and manufacture (Theberge, 1998). By the later nineteenth century, the Coast Survey recognized that creating truly accurate maps depended on determining the underlying shape and size of the earth as well as local and regional deviations; in recognition of that expanded mission, in 1878 the agency was renamed the U.S. Coast and Geodetic Survey (Beall, 1925, p. 1-2).

3As the number of observatories in the country increased as well as skill in the making of ever larger objective lenses, so too did the aperture of observatories’ main equatorial refractors. Often overshadowed in the desire for larger main telescopes, however, was the number and accuracy of observatories’ auxiliary instruments of precision, which were central to the positional programs then dominating astronomy. Although several historians have discussed individual 19th-century U.S. meridian*/transit circles* in the context of a specific observatory or of telescope making more generally (for example, Dick, 2003; James, 1987; King, 1955; Osterbrock, Gustafson & Unruh, 1988; Whitesell, 1999), few have taken a more synoptic view of multiple instruments, their demographics, their history, and their evolution. Because references are scattered, amassing such synoptic information is challenging. One notable attempt was E. G. Martin in 1949, who compiled a list of meridian circles worldwide with apertures greater than 4.5 inches, their apertures, their dates of construction, and their instrument makers (Martin, 1949). Despite a determined effort, he was unaware of two 4.5-inch U.S. instruments (the observatory of Catholic University in Brookland and the Philadelphia High School Observatory at any of its three sites) and Harvard’s instrument appears twice under different names. Moreover, the choice to exclude smaller instruments — which make up over half of the meridian/transit circles in the United States as shown in Table 1 — could be mistaken to imply that the first meridian circle by a U.S.-based maker was a larger one produced in 1888 (the 5.1-inch for Cincinnati Observatory); such an incorrect inference would obscure both the advent of U.S.-based makers fully four decades earlier and the increasing demand for U.S.-built instruments in U.S. observatories, which in the 1890s completely displaced demand for those by European makers. This paper is an attempt to ascertain the complete picture, given the importance of meridian circles to 19th-century U.S. cartography, geodesy, and astronomy, and to establish a taxonomy that illuminates how their development contributed to the rise of U.S. instrument-making hegemony.

Transit Instruments vs. Meridian Circles

4To begin, a few words are necessary about the instruments themselves. Despite their superficial resemblance, the meridian circle and the transit instrument — essentially a meridian circle without the circles — were different instruments. In the United States, they had complementary (albeit somewhat overlapping) purposes. As some aspects of their designs and use merged through technology transfer among U.S. observatories during the course of the 19th century, and both types of instruments operated within a similar assemblage of auxiliary equipment, it is worth being explicit about their respective designs and purposes.

5In its simplest form, a transit instrument was a telescope fixed to a horizontal axis so its movement was confined to observations in a vertical plane. As with a meridian circle, the ends of the horizontal axis rested in Ys so that the telescope could pivot from horizon to zenith*. In a larger transit instrument, counterweights and levers near the ends of the axis relieved most of the telescope’s weight to prevent friction and wear from deforming the pivots. The counterweights also helped minimize flexure of the horizontal axis. There could also be oil or natural gas (or, ultimately, electric) lamps at one or both ends of the hollow horizontal axis to reflect light from an internal diagonal mirror so as to illuminate the eyepiece “wires” or “threads” (commonly of spider silk) or the field of view as needed for the observer’s task.

6Just as for a meridian circle, the vertical plane of choice for the transit instrument commonly was the meridian — the north-south plane* through a site’s local zenith. Unlike a meridian circle, however, a transit instrument also could be rotated 90 degrees and fixed instead perpendicular to meridian in an east-west plane through the local zenith, an orientation called the prime vertical (Chauvenet, 1891/1960, vol. 1, p. 19). In the United States, a meridian circle was always a fixed instrument in an observatory, and it was always mounted in the meridian (hence the name). Transit instruments were often permanently mounted in an observatory as well; most were fixed in the meridian, but a few were (or could also be) used in the prime vertical for specific purposes.

7In the meridian from a well-determined location such as a permanent observatory, both meridian circles and transit instruments could be used to find stars’ right ascension*, or to find the observatory’s true local time. The circles of meridian circles allowed an astronomer to read off stars’ declinations to within 0.05 arcsecond for larger instruments (Chauvenet, 1891/1960, vol. 2, p. 284). Conversely, if using stars with well-determined right ascensions, either a meridian circle or a transit instrument could be used to determine local longitude. If mounted in the prime vertical, a transit instrument could be used to measure either the declination of stars or the latitude of the observing location (Chauvenet, 1891/1960, vol. 2, p. 131).

8Most 19th-century U.S. observatories — especially the smaller ones — were equipped with transit instruments rather than the significantly more precise and more expensive meridian circles; they were used primarily for determining the observatory’s accurate local time or for instructing students. Smaller transit instruments could also be portable for practical astronomy field work (field use is discussed below).

9Determining the latitude of an unknown location was conceptually straightforward: it is the angle between the local zenith and the equator. Given a star whose declination* is well determined in equatorial coordinates, the location’s latitude can be calculated by measuring its zenith distance (angular distance down the meridian from the zenith) at the instant it transits the location’s local meridian, and adding or subtracting the known equatorial latitude of the star (Beall, 1925, p. 47; Wilson, 2010, p. 12). To minimize the effects of atmospheric refraction, pairs of stars were used — one north and the other south of the zenith and within a degree or two of it, each of which transited the meridian within a few minutes of the other. In the field, surveyors or practical astronomers determined the zenith by a plumb bob suspended from the center of the telescope, with the telescope’s legs being adjusted by fine screws until it was exactly horizontal as shown by spirit levels. (In practice, some corrections may be needed because a plumb bob may not hang exactly vertically — i.e., pointing to the earth’s center — because the earth is not a perfect sphere, but an oblate spheroid with local deviations to its shape and distribution of its mass.)

11Portable astronomical transit instruments and zenith telescopes were principal field tools; meantime, meridian circles (or transit instruments) fixed in observatories served as primary geodetic reference locations from which other points could be triangulated and surveyed. In addition, meridian circles in observatories determined and monitored the celestial positions of clock stars and fundamental reference stars that field astronomers repeatedly observed with portable transit instruments for determining the positions of unknown locations.

12Methods of observing were similar for both portable and fixed instruments. In outline, observers timed the instant a star was seen to cross behind or “transit” a vertical wire in the telescope’s eyepiece. In practice, there were usually five or seven parallel vertical wires, and the mean of the timings of the star’s transiting each one was used, often determined with an eyepiece micrometer for greater precision.

1.  Meridian Circles in Nineteenth-Century America

15A few cautions are necessary. The measurements in Table 1 and throughout this paper are the ones that appear in 19th-century U.S. literature. The United States, then as now, did not use the metric system; the American unit of choice was the English or Imperial inch. The inch was not internationally standardized, however, until the 20th century, and the size of the inches of various countries could differ by up to nearly 7 percent. For example, the German optical dynasty of Merz used the French or Paris inch, which equaled 1.0658 English inches, and one French foot equaled 12.7892 English inches (Loomis, 1863, p. 491).

Table 1 - Meridian circles/transit circles installed in 19th-century U.S. observatories (to be consulted in « Document annexe »)

16Some later 19th-century literature overlooked the need to specify which inch was used, resulting in apparent discrepancies in measurements that could be significant in descriptions of larger instruments. All these ambiguities preclude the ability to specify exact metric equivalents. Moreover, contradictions of instrumental measurements in 19th-century U.S. primary literature abound — not only among different sources, but sometimes even on different pages within a single source. In seeking to establish the most authoritative numbers for a given instrument, the greatest weight has been given to observatory annals or other official primary documents (e.g., university catalogues*), or to articles in astronomical publications written by the observatory director or other astronomer.

17Space does not allow discussion of all the meridian/transit circles listed in Table 1. However, the table reveals several important facts worth noting. First, 19th-century U.S. meridian circles were installations for the use of professional astronomers and their students. Although dozens of observatories founded by amateur astronomers — who ranged from casual to serious observers — possessed transit instruments for obtaining accurate local time, none had meridian circles, nor were their research interests (most commonly planetary features, sunspots, variable stars, comets, meteors) ones that would have benefitted from observations made with meridian circles.

18Second, with just three exceptions — the Cincinnati Observatory, the Dudley Observatory, and the Smith Observatory — all the meridian circles were installed in observatories associated with institutions. Five installations were associated with the U.S. Federal government, but the overwhelming majority were affiliated with educational institutions, primarily with colleges and universities. Two of the three exceptions (Cincinnati and Dudley) were initially independent “civic” observatories founded by community members of the cities of Cincinnati, Ohio, and Albany, New York, but within a few decades each became affiliated with educational institutions (the University of Cincinnati and Union College, respectively) (James, 1987; Shoemaker, 1991; Wise, 2004). The Smith Observatory was founded by a wealthy philanthropist William Smith, patron of one of the century’s most prolific comet finders William R. Brooks, in the manner of the wealthy Victorian British patrons (Chapman, 1998); but by 1894 it was associated with Hobart College, where Brooks eventually became professor of astronomy (The Smith Observatory, 1894).

19Third, Table 1 reveals that at least nine of the meridian circles were installed in observatories affiliated with (or soon transferred to) educational institutions that were either co-educational (Haverford College Observatory, Morrison Observatory, Lick Observatory, Carleton College Observatory, and Washburn Observatory) or exclusively for women (Sharon Observatory, Elmira Observatory, Vassar College Observatory, and Williston Observatory). That should not be taken to mean that co-ed or women’s educational institutions specifically intended to produce female professional astronomers, although at least three did (Caroline E. Furness at Vassar College, Charlotte Willard at Carleton College, and Alice Lamb at the University of Wisconsin). Rather, astronomy, mathematics, and other abstract subjects were valued as a means of developing and strengthening the “mental discipline” of both academically inclined genders, somewhat analogous to developing and strengthening muscles. Thus, for most of the nineteenth century — into the 1890s — astronomy was a standard course in the classically based curriculum followed by most liberal arts colleges, taught for at least one semester in the third or fourth year. The course or courses typically consisted of lectures, recitations, textbook exercises, and training in the use of observatory instruments (Marché, 2001-2002).

22To ascertain geographical distribution of meridian circles, the location data in Table 1 were plotted on a map of the present-day continental United States (Figure 1). To avoid undue crowding of dots on the map, no additional dots are shown if more than one meridian circle was installed in an observatory during the 19th century. (Although all the modern boundaries for the 48 contiguous states are illustrated for simplicity of recognition, Arizona, New Mexico, and Oklahoma were not made states until the 20th century. Not shown are Alaska or Hawaii — also not yet states — because neither had any observatory with a meridian circle in the 19th century.)

23The map reveals that most 19th-century U.S. meridian circles were in observatories in the northeast and Midwest regions of the nation, with only a few in the American south or west. At least two observatories (Harvard and USNO) installed two or more meridian circles that operated either simultaneously or in succession. Because of that, the number of 19th-century U.S. observatories owning meridian circles was fewer than the number of instruments.

2.  Makers and Design Philosophies

24The first meridian circle (as opposed to a transit instrument) installed in the New World was the Troughton & Simms transit circle at the Hudson Observatory of the Western Reserve College (now the Western Reserve Academy) in Hudson, Ohio, mounted on September 8, 1838. The instrument was supported on a heavy cast iron frame that rested on a single sandstone pier “entirely detached from the building, and the floor is no where in contact with it”. (Loomis, 1841, p. 44, 45) Thus, by 1838, vibration isolation was understood, and the transit circle was fixed to a pier that had its own separate foundation — a practice that quickly became widely followed in most subsequent 19th-century U.S. observatories.

25In 1844, the observatory of the Philadelphia Central High School in Pennsylvania installed a meridian circle by Ertel of Munich (Loomis, 1856, p. 215-216; Edmonds, 1902, p. 90). “The erection of this observatory formed an epoch in the history of American astronomy”, Hudson Observatory director Elias Loomis asserted in 1850,

in consequence of the introduction of a superior class of instruments to any which had been hitherto imported. It introduced the instruments of Munich fairly to the notice of the American public; and their superiority to the English telescopes was felt to be so decided, that almost every large instrument which has since been imported has been from [German] makers. In the hands of Messrs. [Sears Cook] Walker and [E. Otis] Kendall this Observatory became celebrated, not only in America, but also in Europe (Loomis, 1850, p. 167).

26The ascent of German makers is evident when the information in Table 1 is plotted as a histogram showing the nationality of the maker’s firm for each meridian circle installed in 19th-century America by decade (Chart 1). Although Troughton & Simms (by then under the leadership of William Simms) built several later transit circles for U.S. observatories, that English maker (green) never attained a predominant market share because of the advent of German rivals (orange). French makers (yellow) also never gained a foothold. U.S.-based makers are shown in blue; more on them later. Z indicates the instrument could also function as a zenith telescope.

27The advent of German instruments also introduced German methods of design and observation. In the early 19th century, there had evolved two fundamentally different philosophies of designing and building the best instruments of precision: the English and the German (Newcomb, 1906, p. 342-343; King, 1955, p. 240). Benjamin Apthorp Gould — first director of the Dudley Observatory in Albany, New York, and founder of the Astronomical Journal — had spent three years of graduate study in Europe 1845-1848, including several months each at both the Royal Observatory, Greenwich, and at the Berlin Observatory (Bell, 2007, p. 433; Comstock, 1924). Thus, Gould was able to evaluate first-hand the respective merits of English and German designs. Gould characterized “the one [the English] as the instrument of the engineer, and the other [the German] as that of the artist”. He observed that the English style “is designed for securing absolute uniformity of circumstances in all observations” whereas the German style was designed “for attaining as great diversity of circumstance as is consistent with retaining the same degree of accuracy” (Gould, 1857, p. 404).

28Specifically, the English philosophy sought to attain precision by minimizing error — basically, engineering it out, by keeping everything as fixed and as unchanging as possible. An instrument was cast in one piece, with large and massive measuring circles. The telescope could not be reversed in its pivots, but relied instead on collimators* to ensure that observations made on both sides of the zenith were equivalent. The east-west axis was kept horizontal not by hanging or striding bubble levels, but by periodic observations of the meridian wire as reflected from the mirror-like surface of a pool of mercury* at the nadir*. Microscopes for reading the giant circle(s) were embedded in massive piers. The 4¼-inch transit circle at the Harvard College Observatory (focal length 5 feet), made by Troughton & Simms in London and installed in November 1848, was an excellent example of the English design philosophy (Figure 2). The two circles, engraved with angular measurements, were 48 inches in diameter, the largest of any meridian circle installed in any 19th-century U.S. observatory. They were read with eight microscopes (four are visible on the piers in the figure).

30The German approach was due in large part to the theory of instrumental errors by Friedrich W. Bessel, who effectively “created a new art of observation”, recounted British historian of 19th-century astronomy, Agnes Mary Clerke. In Bessel’s view, every instrument

must be twice made — once by the artist and again by the observer. …Defects that are ascertained and can be allowed for are as good as non-existent. Thus the truism that the best instrument is worthless in the hands of a clumsy observer, became supplemented by the converse maxim, that defective appliances may, through skilful [sic] use, be made to yield valuable results. (Clerke, 1902, p. 122)

31Indeed, Gould quoted Bessel’s asserting that he “could determine the place of a star with a musket-barrel and a cartwheel” (Gould, 1857, p. 408).

32In specifying the design for the Olcott Meridian Circle, completed for the Dudley Observatory in 1856, Gould did something innovative and influential. “…[I]t has been my endeavor”, he wrote, “to incorporate in the design of this instrument the principle, — never before attained, so far as I am aware, — that every instrumental correction, without exception, should be capable of determination by two entirely distinct and independent methods; and in this respect to combine the advantages of the German and the English forms” (Gould, 1857, p. 407). The Olcott Meridian Circle was not only the largest such instrument made for a U.S. observatory at that time, but its 8-inch aperture (focal length of 10 feet) equaled or exceeded the apertures of two-thirds of the equatorial refractors installed as main telescopes in the 25 U.S. observatories built up to then (Loomis, 1856, p. 206-292, 395).

33Meantime, another important trend was building. Although most early 19th-century meridian circles in American observatories were European imports, by mid-century, an increasing number of U.S.-based artisans — many of them German or Prussian immigrants escaping the tumult of several European wars — were fashioning meridian circles and other instruments of precision on American soil (Chart 2). Prominent makers included Camill Fauth, Fauth’s hand-picked successor George N. Saegmuller, William Würdemann (who crafted many instruments for the U.S. Coast Survey), and William J. Young of Philadelphia (Warner, 1985).

34Young built the first 19th-century U.S. meridian circle whose mechanical parts were constructed by a U.S.-based artisan. It was the instrument mounted by 1847 in the Sharon Observatory of the Sharon Female Boarding School in Darby, Pennsylvania (Figure 4) (Sharon, 1847, p. 7-8; Loomis, 1856, p. 258). As shown in Table 1, over the next two decades, Young built additional meridian circles of 3.5 to 4 inches aperture for the observatories at Haverford College in Haverford, Pennsylvania, Alfred University in Alfred, New York (later renamed the Rogers Observatory), and Vassar College in Poughkeepsie, New York.

35In the last half of the 19th century, the two most famous and in-demand telescope opticians — Alvan Clark & Sons from before 1860 to the early 1890s, and John A. Brashear from the early 1880s well into the 20th century (as well as optician Henry Fitz before 1863) — happened to be U.S.-born, as were the mount-builders par excellence Worcester R. Warner and Ambrose Swasey. In addition to the giant equatorial refractors and reflectors that brought them all international renown, the Clarks and Brashear also supplied optics to Fauth & Co., Warner & Swasey, and other mechanicians for meridian circles and other instruments of precision (neither the Clarks nor Brashear made meridian circle mechanical parts). In addition, demand from U.S. observatories large and small was great enough to support a broader ecosystem of additional lesser known or regional U.S. makers of meridian circles and other instruments of precision, including Young and Hiram G. Sedgwick.

36Charting both the apertures as well as the nationality of the makers of meridian circles installed in 19th-century U.S. observatories reveals that in the last two decades of the 19th century, U.S.-based makers nearly exclusively fulfilled the demand for meridian circles in U.S. observatories — even for the largest instruments (greater than 6 inches aperture), which had remained the province of German makers through the 1880s (Chart 2). Indeed, the last large, new (as opposed to refurbished) German meridian circle installed in a U.S. observatory was the 6.4-inch (focal length 76 inches) by Repsold for Lick Observatory atop Mount Hamilton, California, in 1884 (Table 1).

3.  Meridian Circles within Innovative Assemblages

37Although Greenwich had been established as the British prime meridian in 1721, through much of the 19th century Paris, Berlin, Cadiz, and cities in other countries also served as prime meridians for their own nations. It was not until the International Meridian Conference in 1884 that the Royal Observatory at Greenwich, England, was voted to be longitude 0 degrees, the reference longitude from which every location around the planet should be determined as well as international time zones (Bartky, 2000) although some nations did not accept that for decades (Black, Gebel, & Newton, 1984, p. 388). Even so, given the vastness of the Atlantic Ocean, into the mid-19th century there was also serious discussion whether there should be a separate American prime meridian (Dick, 2003, p. 124-127; Waff, 2006). Indeed, a provisional American prime meridian was established using the meridian of the center of the dome of the old U.S. Naval Observatory (Beall, 1925, p. 3).

38So by the early 19th century, astronomers had developed a number of after-the-event methods of determining approximate longitudes either for mapmaking or for navigation. Several involved the moon, such as comparing the local timings of the contacts of a total solar eclipse from various locations along the path of totality, or of occultations (eclipses) by the moon of specific stars listed in the Nautical Almanac, or of moon-culminating stars (stars that may transit the meridian at about the same time as the moon). But all the methods using the moon suffered from the same weakness: the moon’s motion — which is mathematically complex, as it feels the gravitational pull not only of Earth but also of the Sun — was only imperfectly known. As a result, the various astronomical techniques gave inconsistent results. And in the early and mid-nineteenth century, where different maps met, meridians of longitude — especially measurements of transatlantic longitude — differed from one another by several seconds of time, corresponding to a discrepancy of up to a mile (G. P. Bond, 1850, p. 2; Gould, 1869, p. 3-6).

3.1.  Chronometric Longitudes

39By the 1820s, it became clear to a number of European astronomers and cartographers that good marine timekeepers could be used to determine longitudes of fixed locations on land as well as transient locations of ships at sea. In 1821, Danish-German astronomer Heinrich Christian Schumacher (founding editor of Astronomische Nachrichten) first used chronometers to determine the difference in longitude between two points (Copenhagen, Denmark, and Hamburg, Germany) nearly 200 miles apart within a second of time, corresponding to a longitude error on the order of 1,000 feet (Struve, 1844, p. 1). Both the British Admiralty and the Russian Emperor Nicholas I underwrote the costs of additional major chronometric expeditions. By the mid-1840s, European chronometric expeditions had revised the longitudes of key points from Moscow to Valentia, Ireland — an east-west distance exceeding 2,000 miles (Chronometer–Expeditionen, 1846; Airy, 1847; W. C. Bond, 1856, p. cxxxix).

40“It therefore has now become our duty to complete the chain which shall unite us more intimately with the Observatories of the Old World”, wrote William Cranch Bond, founding director of the Harvard College Observatory in Cambridge, Mass., and also head of longitude operations for the U.S. Coast Survey (W. C. Bond, 1856, p. cxxxix). In early 1849, Bond interested John Hartnup, founding superintendent of the Liverpool Observatory (built in 1843), in a grand chronometric expedition with the Liverpool Observatory (built 1843) as the eastern end point and Harvard’s newly installed (November 1848) transit circle as the western end point for spanning 3,000 miles across the Atlantic Ocean. (Schmidt & Dearden, 2020, part 2) Alexander Dallas Bache, superintendent of the U.S. Coast Survey, allocated funds for renting and insuring three to five dozen chronometers and paying their cost of passage multiple times in both directions, to begin that current (1849) sailing season.

42Altogether, the Harvard and Liverpool observatories recorded 1,065 exchanges of chronometers — greater than any previous or later chronometric expeditions (Knox, 1957, p. 559). Because its position was so minutely determined, the Harvard College Observatory served as a de facto American prime meridian until the first successful direct measurement of the longitude between North America and Europe by transatlantic cable in 1866 (Gould, 1869). Chronometric methods of determining longitudes remained a standard tool for the Coast Survey well into the early twentieth century for astronomical, cartographic, and exploring expeditions to remote regions (Hayford, 1899).

3.2.  Telegraphic Longitudes

43Until the advent of the telegraph in the 1840s, however, there was no method by which astronomers could simultaneously compare the local time on a clock at an unknown location with the time on a clock at a reference meridian (such as a fixed observatory). Samuel F. B. Morse’s invention and improvement of the telegraph in the late 1830s and early 1840s bore the promise of a powerful new assemblage of technologies for directly comparing clocks hundreds of miles apart with “the same degree of precision as if they were placed side by side” (Loomis, 1856, p. 310) in what today would be called real time.

44Such comparison of clocks was first tried on the experimental line between Baltimore and Washington, D.C., on June 9, 1844 — just two and a half weeks after Morse’s inaugural message “What hath God wrought?” Thereafter, the telegraphic method of determining differences in longitudes rapidly evolved. As soon as commercial telegraph lines linked offices in major cities, the Coast Survey paid to have additional wires extended to astronomical observatories; the agency then contracted with local astronomers to use an observatory’s own meridian circle or transit instrument, or sent trained observers equipped with the agency’s own transit instruments. Within the first few years, three techniques were devised for telegraphically comparing local times using sidereal* and/or mean* solar astronomical clocks* and chronometers (exchange of clock signals, method of coincidences, and exchange of star signals); they worked with varying degrees of rigor, but became standard to use in tandem. By the end of 1848, Coast Survey superintendent Bache reported that the telegraphic method of determining longitude “may be considered to have passed into one of the regular methods of geodesy” (Bache, 1848, p. 19).

45However, Bache’s deputy in charge of telegraphic longitude determinations, mathematician Sears Cook Walker, was troubled by the lack of a permanent record of a longitude determination (Mitchel, 1860, p. 235). The history of the printing chronograph* is murky with controversy over priority of invention and simultaneous invention and competing designs, which is beyond the scope of this paper and has been discussed elsewhere (Evans, 2002). But by 1850, Harvard observatory director Bond and his sons had developed a cylindrical drum chronograph, possibly somewhat influenced by earlier cylindrical design attempts (e.g., see Saff, 2019, p. 111ff). Ultimately, the Bond “spring governor” cylindrical drum design became standard for printing chronographs used in most 19th-century U.S. observatories.

46With the addition of a method of permanently capturing a reliable record of observations, the Bonds’ assemblage for the telegraphic method of determining longitudes was essentially complete. The telegraphic method won immediate accolades and adoption on both sides of the Atlantic, and rapidly became known as the “American method of longitudes” in contemporary literature (Airy, 1849; Stachurski, 2009, p. 87-140). The telegraphic method of determining longitude was important for eight decades, being replaced only in the 1920s by radio positioning techniques.

4.  Revolutionizing Observatory Practice

47Significant for purely astronomical research with meridian circles — such as measuring stellar parallaxes and proper motions for investigating larger celestial questions such as the structure and dynamics of stars in space — telegraph technology also revolutionized nightly data recording within the walls of a single observatory, regardless of whether or not that observatory had telegraph connections to the outside world. By 1850, this method became adopted and known on both sides of the Atlantic as the “American method of transits”.

48In the American method of transits, telegraph equipment was installed in all the observing rooms, to function effectively what today would be recognized as a local-area network. It provided a permanent written record of clock beats and nightly observations more reliably and with greater accuracy than the long-standing eye-and-ear method, which required mentally interpolating the audible beats of a clock with the passing of the star behind a wire and handwriting the result (Airy, 1849, p. 27).

49In 1859, the Bonds installed a switchboard in the Harvard College Observatory that allowed the time signals from a central clock to be recorded along with observations from various telescopes. The switchboard was improved and simplified in 1871 by director Joseph Winlock (Figure 5). It allowed astronomers to switch easily from telegraphic longitude operations to recording the observatory’s own regular nightly astronomical observations. The figure shows the switchboard from both above (most of the image) and the side (far right). Six pairs of wires (top) could be connected through four switches (middle plate) to link any combination of telescope, clock, and chronograph. The system also could transmit time signals to regulate clocks of Boston jewelers and other merchants.

50The American method of transits dramatically improved observational speed, efficiency, and productivity. Tapping a telegraph key was so fast that observers could record more observations per minute of a star transit, eliminating the need for multiple nights of observing the same stars to attain the same statistical precision. Thus, the vertical wires in a telescope eyepiece reticle could be much more closely spaced: instead of being 15 seconds apart (so that 15 seconds elapsed as a star drifted from one wire to the next, allowing time for an observer to note the time and write it down), they could be spaced just 2 seconds or even 1.5 seconds apart. In Walker’s words, “fifty wires may take the place of seven, and one month’s work may take the place of a year’s work” [his emphasis] (Walker, 1849, p. 213). Despite Walker’s enthusiastic estimate, 25 wires seemed to become most widely adopted.

51The gain in observational productivity — a boon especially for creating star catalogues — was estimated to range from a factor of seven or ten, to 36, to even as high as 70 (Gould, 1853, p. 257). Telegraph technology and chronographs continued to be used by visual observers in conjunction with meridian circles and transit instruments into the early 20th century.

5.  Astronomers and Artisans: A Technology-Transfer Ecosystem

52Although many 19th-century philanthropists funded the founding of a new observatory building and main telescope, funding lagged for most observatories — even some larger ones — for specialized equipment, maintenance, and adequate staff for research (a complaint of physicist Henry A. Rowland in 1883 (Rowland, 1883)). That situation compelled astronomers either to make difficult choices or to display great inventiveness. As an example of choices, in 1846, Cincinnati Observatory director Ormsby MacKnight Mitchel heeded Airy’s counsel of four years earlier not to undertake any regular series of meridian observations, but to focus the “whole of your observing energies” in using the 11-inch Merz equatorial refractor (Airy, 1846, p. 3), largest in the young nation when it was installed in 1845. Thus, Cincinnati’s first meridian circle was not acquired until 1867, five years after Mitchel’s death.

53One economy followed by at least five college observatories after 1880 (see Table 1) was simply to acquire a very small (aperture 3 inches) transit/meridian circle, which was entirely adequate for determining local solar or sidereal time and instructing students, especially if original research was not a priority. Another approach was to improvise the functions of a meridian circle with a different type of instrument, as was done at the Washington University Observatory in St. Louis, Missouri, which mounted in the meridian an altitude-and-azimuth instrument equipped with 2-foot circles (Snow, 1898, p. 146).

54Alternatively, Mitchel at Cincinnati and later George Washington Hough of the Dudley Observatory experimented with devising and using a prototype “declinometer”, an electromechanical addition to give Dudley’s 6.4-inch Pistor & Martins transit instrument (or any astronomical instrument fixed in the meridian) the capability of measuring differences in zenith distances between two celestial objects (Mitchel, 1852; Hough, 1866, p. 22, 44-56).

55More commonly, astronomers invented and/or worked with instrument makers to devise more economical or faster ways that transit instruments could be adapted or outfitted to measure declinations or latitudes in addition to right ascensions, even if the precision was not quite as great as that attainable with an actual meridian circle. One technique, independently invented in the 1850s by Chester S. Lyman of Yale College and George Davidson of the U.S. Coast Survey, was to equip a finding circle on the horizontal axis to yield readings (or to add a second, more finely graduated reading circle, some capable of reading to a fraction of a second of arc). They also added a declination micrometer and a fine level. The modifications allowed a transit instrument to determine zenith distances or latitudes with precision similar to a zenith telescope (Lyman, 1860; Davidson, 1869). By 1883, Fauth & Co. in Washington, D.C. was offering such an option in its commercial catalogue (Fauth & Co., 1883, p. 22-23).

56Such adaptations of transit instruments to obtain some of the functionality of meridian circles (or zenith telescopes) made their way into major observatories. For example, the 4.1-inch transit instrument of the Lick Observatory, made by Fauth & Co., was installed in 1881. Although it was invariably called a “transit instrument”, it was equipped with two circles 16 inches in diameter (Figure 6). In 1885, it was remodeled by the makers to include sensitive spirit levels and other modifications so “it can be used either in R.A. or Z.D.” and to afford “a check on any system of standard declinations” (Holden, 1887, p. 68, 69).

57In addition, the closely connected network of astronomers and instrument makers was an intangible assemblage of its own, facilitating fruitful sharing of design concepts. 19th-century U.S. astronomers frequently visited other observatories before ordering instruments for their own, and makers readily customized instruments. Influences were geographically widespread and freely acknowledged.

58For example, the Lick Observatory 4.1-inch Fauth transit instrument was “essentially of the same pattern as the meridian circle of the School of Science at Princeton, New Jersey, by the same makers” (Holden, 1887, p. 68), meaning the John C. Green Observatory at the College of New Jersey at Princeton, mounted in 1877 (see Table 1) (Baird, 1878, p. 30). In turn, the Fauth transit mounted at the Chabot Observatory in Oakland, CA, was “an exact duplicate of the one at the Lick [Observatory]” (Campbell, 1887, p. 38).

59Presuming astronomers were not being unduly casual in their technical terms, this cross-fertilization of design features and capabilities seems to have given rise to at least a partial merging of designs, somewhat muddying the distinction between types of instruments (Table 2). Indeed, by the late 19th century, one observatory directory noted that the 3-inch Fauth transit circle in the observatory of Creighton College in Omaha, NE, “serves the triple purpose of transit instrument, meridian circle, and zenith telescope” (Chandler, 1900, p. 32).

Table 2 - Instruments called “transit instruments” although they had circles

Called	Year	Observatory	Affiliation	Location
				City	State
TI	1853	Obs. of U of State of MO	U of Missouri	Columbia	MO
TI	1863	Dudley Obs. (1st & 2nd sites)	n/a	Albany	NY
TI/ZT	1881	Lick Observatory	Univ. of California	Mount Hamilton	CA
TI/ZT	1881	Students' Observatory	Univ. of Wisconsin	Madison	WI
TI	1886	Chabot Obs (1st & 2nd bldgs)	Oakland School Dept.	Oakland	CA
TI	1889	Observatory	Iowa College	Grinnell	IA
TI/ZT	1897	Observatory (2nd site)	U of Illinois	Champaign	IL
TI/ZT	1897	Charles Smith Scott Obs.	Park College	Parkville	MO
TI/ZT	1898	Case Observatory	Case Institute of Applied Science	Cleveland	OH

Called	Dimensions		Circles				Makers
	Ap.	F.L.	Diam	No,	Microscopes	Verniers	Lens	Mechan.
TI	2,0625	23	10,5	1	x	x		Brunner of Paris
TI	6,375	96	24	2				P&M
TI/ZT	4,1		16	2		x	AC&S	Fauth
TI/ZT	3	38	12	2				Fauth
TI	4,125		16	2		x		Fauth
TI	4		16	2				Fauth
TI/ZT	3	37	12&12.5	2		x	Brashear	W&S
TI/ZT	3	37	12&12.5	2		x	Brashear	W&S
TI/ZT	3	37	12&12.5	2		x	Brashear	W&S

Source: the author, from the same sources listed for Table 1

6.  Conclusion

60As the 19th century progressed, and as the geographical center of U.S. population moved west, so too did observatories and their meridian circles. When the observatory movement began in the early 1840s the center of population of the United States was in West Virginia, but by the beginning of the Civil War it had moved to a location just north of Cincinnati, Ohio, ending, in 1900, in south central Indiana (U.S. Census Bureau, 2021). Similarly, from 1838 through 1866, 18 of the 20 meridian circles were installed in east-coast states, except those in the observatories in Hudson and Tuscaloosa. But following the Civil War, Americans, liberated from a north-south dichotomy, started thinking of themselves less as a collection of individual states, and more as a unified nation. After the completion of the first transcontinental railroad in 1869, many committed themselves to an expanding, dynamic republic. Reflecting that, Table 1 reveals that in the 1870s two of the five meridian circles installed found homes in Midwest or western states, and in the 1880s, joined by nine of 15 more — their observatories settling westward with the population.

61American astronomers and artisans also made the meridian circle their own by adaptations, improvements, and customization of their design. As Charts 1 and 2 make plain, by the 1890s U.S.-based instrument makers came to dominate in the production of the instrument for U.S. observatories. Meridian circles were an instrument of the professional astronomer, although many of the smaller instruments served to instruct students in practical astronomy.

62Furthermore, envisioning the taxonomic Tables 1 and 2 as dynamic over time leads to a heuristically useful evolutionary conceptualization. The advent, adoption, and modification of meridian circles in 19th-century America seem to be analogous to an invasive species that, once planted in the New World, spread throughout their new environment, adapting as necessary to new niches through the agency of their new construction. As such, their development seems to exemplify an evolutionary system as documented in George Basalla’s The Evolution of Technology (Basalla, 1989).

63Few 19th-century U.S. meridian/transit circles remain extant in working order today, however, a notable exception being the one at the Detroit Observatory. Some others survive as historical exhibits on display, such as the ones at the Chamberlin Observatory, Hopkins Observatory, and Hudson Observatory. An undetermined number elsewhere have been dismantled and put into storage, or even deaccessioned or demolished. But their legacy remains in both countless publications and in the firm establishment by the century’s end of U.S. mechanical as well as optical precision skill.