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  The American Surveyor     

Revisiting Its Past: Installing David Rittenhouse's Transit Telescope to the Outside of Philosophical Hall Print E-mail
Written by Jeff Lock   
Saturday, 27 May 2017

A 2.731Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

After having completed several restorations on instruments owned by the American Philosophical Society, I was appointed to the Committee on Museum to review the condition of current instruments in its collection and to contribute to the vetting process of new acquisitions.

After the 2015 American Philosophical Society Committee on Museum meeting, Chairman Keith Thomson brought me to a south-facing room on the second floor of the museum (see Figures 1, 2, and 3) to discuss what a bracket assembly may have been used for that has been in place outside the room's window for around 200 years.

We agreed that the bracket was most likely used for a telescope to view the south meridian line, and we speculated that the bracket assembly could have been used for the installation of the David Rittenhouse Telescope in the collection of the APS Museum. The initial measurements showed that it was possible that the Telescope fit into the window bracket assembly, but further investigation and restoration would be required to confirm our hypothesis.

The initial problem was the fact that the original trunnion bearings were sheared off (see arrows in Figures 6 and 7) and the sliding blocks securing the bolt assemblies were frozen from lack of use for quite some time. Luckily the bolts and block assemblies were painted with the window frames over the years and the paint had protected the bolts and nuts from excessive corrosion. (Figure 4)

A colleague and friend of mine, Carl Guckelberger, had secured an acetylene tank to carefully heat the nuts, to release them without damaging the threads (Figures 5 and 6). After 45 minutes of careful heating to prevent distortion, we were successful in releasing the first nut (Figure 7). The same process was repeated on the second nut and, once released and cooled, the nuts turned smoothly by hand.

With the brackets removed, I brought them back to my workshop to drill out the pins and remove the brass tenons from the iron mortise assemblies (Figures 9 and 10). I then began to design and fabricate the trunnion bearing housings to the dimensions of the trunnion bearings on the David Rittenhouse Telescope. One important modification I decided to incorporate for safety reasons was the design of the housings into which the bearings were installed. I felt that the originals would have had a "drop-in" V design similar to a Meridian Transit cradle (Figure 10A).

I decided to modify this design to a totally encapsulated, circular fabrication to lessen the chances of the telescope being dropped from the second story window (Figure 10B).

This enabled the telescope to be installed into one housing first, then the second housing attached to the other bearing, and the bracket plate laid onto the window support assembly, then tightened into place.

The final proof came the day before the 2016 APS Committee on Museum meeting. Carl Guckelberger, Ron Hoppes, and I were accompanied by Mary Grace Wahl (Project Director of Collections Care and Management at the APS) with their David Rittenhouse telescope that was also used for the 1769 Transit of Venus.

Prior to the installation, we met with Karie Diethorn, the Chief Curator, Independence National Historical Park, on the southern grounds of Independence Hall to install a meridian line south of the particular window on the APS building. Carl, Karie, and I used our iPhone compasses to reach a common N ­ S meridian line. We then placed two stakes into the ground 20 feet apart with a yellow caution tape stretched between them (see arrow in Figure 11).

Once we were satisfied with the accuracy of the meridian line, we returned to the window to install the telescope. Prior to assembling the telescope, we noticed that both the left and right sides of the window bracket were not square to the wall but angled, pointing exactly at the distant meridian mark (Figure 11).

The process could not have gone any smoother. Once the telescope was installed (Figures 12 and 13), we were able to make minor adjustments by sliding the iron blocks over the window support to perfectly align the telescope to the meridian mark one hundred yards away.

After the installation, we realized that the open-end bearing housings could be problematic due to the fact that, if one trunnion bearing would be inserted as far as possible on one side, it would allow the other side to disengage and be free of the bearing housing. To remedy this situation, I decided to install two radius-knurled end caps to keep the end float of the trunnions within the bearing housings (Figure 14).

The final proof of the combination of the David Rittenhouse Telescope and the APS window bracket is that the clearance between the forward protractor bracket on the telescope and the southern bar of the window bracket with the Telescope in the horizontal position is 1/16." (See arrow in Figure 15)

It was very rewarding for all of us involved in the project to have the telescope and bracket assembly fit so accurately together as a unit, aligning the telescope directly to the north/south meridian line. This would have been used to determine the accurate time of 12:00 noon when the sun would have bisected the cross hairs in the telescope and the clocks in Philadelphia would have been accurately set using this procedure (Figures 16 and 17).
Jeffrey Lock has been involved in the field of antique restoration for 40 years. During the last 20 years he has specialized in surveying instruments from the 17th and 18th centuries, primarily the instruments of the American Colonial period. He has completed extensive research at major institutions both in the United States and England and was a guest lecturer at the Museum of the History of Science, Oxford, in the Masters Program. Additionally, he was an appraiser for the Antiques Roadshow in Science and Technology.

A 2.731Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

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