John Bird Transit Telescope Cradle and Tripod Fabrication


Charles Mason and Jeremiah Dixon arrived in Philadelphia on November 15, 1763. Over the next five years they established locations and boundaries to seconds-of-arc accuracy, a remarkable feat in the Old World and unprecedented in the New. They brought with them two major instruments, both crafted by London’s John Bird, the foremost instrument maker of the time.

The first instrument, a six-foot Zenith Sector, was used to establish latitude by measuring the positions of stars in the meridian close to the zenith. John Bird stated that the accuracy of this sector was two seconds of arc. Unfortunately, this instrument was lost in a fire in 1897.

The second instrument was the John Bird Telescope and Hanging Level that was configured as a portable tripod-mounted transit and equal altitude instrument, the subject of this article. Using star tables provided by both Paris and Greenwich observatories and a seconds-beating clock, they used it as an equal altitude instrument to establish true north by timing the passage of selected stars crossing the meridian in the usual way.1

The survey of the Pennsylvania/Maryland border, Mason and Dixon’s “West Line,” followed a constant line of latitude. Its survey required turning azimuth angles westward from their meridian to seconds-of-arc accuracy. No field azimuth instrument of this accuracy existed in that era. The team used their knowledge of astronomy, the Bird transit instrument and their clock to lay out successive segments of a great circle which were then adjusted to conform to a “lesser circle” of fixed latitude. The bearing for the great circle segments had been calculated to the nearest second of arc by Mason using spherical trigonometry. They noted the times that selected stars (those which would be setting just north of west) crossed their meridian. They then calculated the times when these same stars would arrive at the desired westerly bearing. Having turned the instrument to the west and tracked each star with the vertical crosshair, they clamped the azimuth at the correct instant of time and recorded the direction by placing pegs in the ground approximately 1/2-mile west of the instrument. The mean position of the pegs was taken as the correct bearing. They proceeded to survey westward along this bearing and verified, using the zenith sector, if the western terminus was at the correct latitude for each segment. In this manner, Mason and Dixon laid out the 233-mile long “West Line” in short segments, each great circle segment of bearing 89 degrees, 55 minutes, 51 seconds west of north.2 It’s clear from reading the Mason & Dixon journal that they thoroughly enjoyed the intellectual challenge and rewards of the task before them.3

Fortunately, the John Bird Telescope and Hanging Level had been preserved for posterity at Independence Hall in Philadelphia. In 2008, Steven Turner, Curator of the Division of Science and Medicine at the Smithsonian Institution, and I scheduled an appointment with Independence Hall’s Chief Curator Karie Diethorn to inspect the sole surviving instrument used by Mason and Dixon in the establishment of the boundary line between Maryland and Pennsylvania.

It was apparent from our initial inspection that the instrument had been re-tasked, most likely for the use of establishing the noontime meridian for setting clocks in Philadelphia. The optical modifications were obviously 19th century, as well as the blackening of sections of the brass, a process that was in vogue at that time. It had also been polished over the years, as witnessed by the amount of residue existing on the instrument.

It wasn’t until 2014 that my work with the Bird instrument commenced, after David S. Thaler, PE, LS, and Chas Langelan, LS, organized a national fundraising campaign and approached Independence Hall with the concept of conserving the telescope and trunnions4 and reproducing a working tripod, protractor and cradle assembly similar to what Mason and Dixon would have used in the field on their line.

Diethorn approached the American Philosophical Society in Philadelphia to request the loan of a tripod that I constructed in 2012 to be used as the base assembly to mate up with an accurate recreation of a John Bird cradle assembly, custom-built to hold the Independence Hall John Bird telescope and hanging level. The tripod5 and a corresponding cradle were originally commissioned by the American Philosophical Society to support its David Rittenhouse Transit Telescope, used in the Transit of Venus of 1769 and put on display at the APS to celebrate the 2012 Transit of Venus.

To complete the tripod reproduction, I studied several examples of transit telescope tripods and cradles in the collections of the Smithsonian Institution, and a tripod example by David Rittenhouse at the Historical Society of Montgomery County in Pennsylvania. These examples were built by craftsmen who, almost certainly, viewed and emulated design elements of the Bird portable instrument used by Mason and Dixon. With the permission from the American Philosophical Society for use of their reproduction tripod secured, I began work on the recreation of the Independence Hall Bird transit’s protractor and cradle assembly.

Due to its rarity and historical importance, the Bird telescope and hanging level could not be sent to me for measurements, alignment and fitting to be completed in an expeditious manner. I made several visits to Philadelphia as the project progressed, but before my work could begin in earnest, I sought out other examples of Bird’s work.

I first approached Harvard University’s Sara Schechner, Ph.D., David P. Wheatland Curator of the Collection of Historical Scientific Instruments, to secure archival photographs of a similar John Bird transit instrument, which unfortunately is no longer in its collection due to its theft while on display in 1979. With Schechner’s gracious help, I was able to begin fabrication of the cradle assembly as accurately as possible.

Schechner further recommended that I contact an acquaintance of mine, Dr. Michael Korey, whom I had met while visiting MIT to address its Scientific Instrument Commission. Korey is the Curator of the Mathematisch-Physikalischer Salon, Dresden State Art Collections in Dresden, Germany, and another Bird transit instrument was among his collection. Korey’s assistance in my recreation was instrumental and his colleague Lothar Hasselmeyer dedicated considerable effort to capturing excellent close-up photographs and detailed measurements.

The Harvard and Dresden instruments provided excellent detail for the recreation of cradle. However, it should be noted that both instruments’ pedestals were configured for stationary observatory use and therefore contributed little to the portable tripod design.

The complexity of John Bird’s work was typical of the highest level of craftsmanship from the mid-18th century and is extremely challenging to duplicate today. The cradle construction was built out of three primary pieces, two vertical and one horizontal. I followed the original construction techniques to the letter. The vertical stanchions were fabricated in an “L” shape out of one solid piece of brass, without a joint at the 90-degree junction. The same holds true for the horizontal central support with an integral female dovetail joint at the bottom center, which was also fabricated out of a single piece of brass.

As the fundamental cradle started to take shape, I carefully studied the detailed photos from Dresden of its more intricate elements including the ingenious micrometer assembly, which was used to subdivide the degree readings. “Micrometers of this pattern were to become standard features of all Bird’s instruments, and exhibited a significant advancement on the screw-edge micrometer used by Flamsteed … During this operation, the Bird micrometer was disengaged, and only turned at the critical moment of observation, reducing wear and friction to a minimum.”6

To accurately fabricate the protractor assembly, I was also able to study close-up photographs supplied from Schechner and Korey, allowing me to engrave the numbers in the same style as Bird.

To increase the accurate functionality of the tripod assembly, a tangent arm needed to be added the APS tripod. This addition would have been utilized in the field and would clamp to the center shaft supporting the cradle. A similar tangent arm example can be seen in the two observatory stands in the photographs of the Bird instruments from Harvard and Dresden. After studying existing examples, I decided to model the recreation after a similar tangent arm assembly, as seen on the Andrew Ellicott tripod in the collection of the Smithsonian Institution.

The tangent arm permitted smooth, precise adjustments to the telescope’s azimuth pointing direction. Modern instruments have similar tangent adjustment screws to accurately adjust the azimuth scale. The Bird instrument had no such azimuth scale, only the smooth adjustment used to align the reticule’s vertical crosshair to an object such as a star. To compensate for the lack of an azimuth scale, Mason and Dixon would have measured the time of a star’s transit from which the azimuth angle is calculated, as described above.

The tangent arm also incorporates a clamping arrangement for securing the arm to the vertical spike, so the telescope can be rotated through large azimuth angles and then re-clamped for precise tangent adjustment when making equal altitude measurements or the West Line bearing measurements.

With the project completed, I eagerly anticipate the John Bird Telescope and Hanging Level returning to its home on display at Independence Hall following the unveiling of its full restoration at a 250th anniversary celebration, held in honor of Mason and Dixon in Baltimore in October, 2015. For myself, it was a real honor to work with the instrument, gaining an understanding of its complexities and learning a great deal from the mastery of John Bird’s design.

Many thanks go to Karie Diethorn who continually arranged her schedule to be available during my visits and for her extremely valuable input throughout the entire project. Also, my gratitude goes to her two assistants, David Mitchell and Nicole Altmann. Their assistance during my visits, attention-to-detail, and willingness to help were greatly appreciated.

I would also like to thank three colleagues who assisted me on site in this effort. First, Paul Temple, Ph.D. for many enjoyable discussions about how this instrument was used by Mason & Dixon and Ron Hoppes who lent his personal workshop for some of the critical alignment fabrication on the cradle. Finally to Carl Guckelberger for precision measurements that were needed to accurately fabricate the cradle components.

My thanks go to David Thaler whose initial concept and follow-through with this project allowed the Mason and Dixon instrument to be placed in an environment where it will receive the attention its historical significance deserves.


To those interested in the specific technical details of Mason & Dixon’s procedures, I would refer the reader to Ed Danson’s book, Drawing the Line: How Mason and Dixon Surveyed the Most Famous Border in America. 7


1 Mason, A. Hughlett. Journal of Charles Mason and Jeremiah Dixon. Memoirs of the American Philosophical Society, Vol. 76. Philadelphia: American Philosophical Society, 1969, Page 90

2 ibid, page 73.

3 ibid, page 130 “This day from the Summit of Sidelong Hill I saw the Line still formed the arch of a lesser circle very beautiful, and agreeable to the Laws of a Sphere.” July 6, 1766

4 The conservation work was expertly performed by Adam Jenkins.

5 I wish to give credit to the masterful patination job that was completed on the wood elements of the tripod by restoration specialist and good friend, Mary Miles of New York, N.Y.

6 Chapman, Allan. Dividing the Circle. Page 74

7 A revised and updated second edition of Edwin Danson’s book, Drawing The Line:  How Mason and Dixon Surveyed the Most Famous Border in America (John Wiley & Sons, New York) is due to be published in March 2016.  The procedures for observing equal altitudes, determining local sidereal time and deriving an astronomical azimuth with Bird’s instrument are described, with examples.