A transit instrument is simply a refracting telescope mounted on a horizontal
axis. A transit and equal altitude instrument is a transit instrument that is
equipped with a graduated vertical circle or arc. The French geodesist Pierre L.
M. de Maupertuis took a portable instrument of this sort on his geodetic
expedition to Lapland in 1736–1737, and used it to define the local meridian,
and to regulate the clock by taking equal altitudes of the sun before and after
noon; this instrument was made by George Graham in London. A few years later,
Pierre C. Le Monnier installed a similar instrument in the Paris Observatory;
this one was made by Jonathan Sisson in London.
John Bird (1709–1776), a London maker who had perfected his skills working
with both Graham and Sisson, made several transit instruments, both fixed and
portable. In 1763, in preparation for Charles Mason and Jeremiah Dixon’s
historic survey of the boundary between Maryland and Pennsylvania, Thomas Penn,
the Proprietor of the Commonwealth of Pennsylvania, purchased an instrument of
this sort. Mason and Dixon described it as "an equal altitude or transit
instrument (for it was contrived so as to serve either purpose at pleasure),
made by Mr. John Bird, of the same construction with that described by M. Le
Monnier, in the preface to the single volume of the French Histoire
Celeste." This instrument was used for astronomical and geodetic purposes.
The astronomers who gathered in Philadelphia to observe the transit of Venus
across the face of the Sun in 1769 used it to regulate their clocks. Andrew
Ellicott and other American surveyors used it to extend the Mason-Dixon line to
the western edge of Pennsylvania in 1784. And in 1786, Ellicott ran the last 55
miles of the western boundary of Pennsylvania up to Lake Erie "by a most
excellent transit instrument, made by Mr. Bird, and which had been used by
Messrs Mason and Dixon, some years before in this country." In 1912,
remnants of this instrument were found beneath the flooring in Independence
Hall. They are now owned by the City of Philadelphia, and on loan to
Independence National Historical Park.
Two American copies of this Bird instrument are now in the National Museum of
American History. One was made by Andrew Ellicott for his own use in 1789. The
other was made by Henry Voigt in 1804, and used by Isaac Briggs, Surveyor
General of the Mississippi Territory. There were other American transits as
well. In order to regulate his clocks during the 1769 transit of Venus, David
Rittenhouse made a transit instrument described as being "of as simple a
construction as I could contrive." This is now at the American
Philosophical Society. And in 1798, Ellicott’s brother Joseph asked their
younger brother Benjamin to build a similar instrument.
Ref: S. A. Bedini, "The Transit in the Tower: English Astronomical
Instruments in Colonial America," Annals of Science 54 (1997):161–196.
