scope was placed exactly in the meridian; a fine silver plumb line and a 

 glass plate ruled with vertical and horizontal lines were placed immedi- 

 ately in front of the photographic plate; and the distance between the 

 objective and the plate was measured as nearly as possible. Although 

 none of the observations yielded a satisfactory value for the solar parallax, 

 the American photographic method was deemed the most successful. The 

 equipment was used again for the Transit of Venus of 1882, and for 

 many other astronomical expeditions outfitted by the U.S. Naval Ob- 

 servatory. Some pieces of this equipment, only recently retired, have been 

 given to the Smithsonian Institution (q.v.). 



The 5-inch visual refracting telescopes were used for observing con- 

 tacts and occultations of stars by the moon. They were equatorially 

 mounted, and furnished with divided circles, clock drive, and double- 

 image micrometer. Like the telescope the Clarks had made for Dawes in 

 1859 (q.v.), these were adjustable for any latitude. The chronographs 

 were regulated by a Hipp spring, supposedly more reliable in the field 

 than the Bond spring governor. These 8 telescopes, like the photographic 

 apparatus, were for many years taken on astronomical expeditions around 

 the world. In 1878, for instance, Alvan Graham used a Dallmeyer por- 

 trait lens, held on one of the Transit of Venus equatorial mounts, to 

 photograph a total solar eclipse (see above, p. 33). 



Etienne Leopold Trouvelot, whose astronomical observatory was 

 located in Cambridge, Massachusetts, took good advantage of his neigh- 

 bors. In preparation for his manual of astronomical drawings Trouvelot 

 made observations with the U.S. Naval Observatory 26-inch telescope 

 (q.v.), as well as with the 26-inch for the University of Virginia (q.v.), 

 while they were still in the Cambridgeport workshop; and he frequently 

 compared observations with Alvan Graham. Although without a Clark 

 telescope of his own, Trouvelot did possess a Clark spectroscope with a 

 diffraction grating ruled by Lewis Morris Rutherford. 217 During the 1878 

 solar eclipse Trouvelot used a small Merz refractor with a solar eyepiece 

 and Barlow lens made by the Clarks. - ' 18 



The U.S. Army Battalion of Engineers operated an astronomical 

 observatory in conjunction with their Engineer School of Application at 



217 Etienne L. Trouvelot, The Trouvelot Astronomical Drawings Manual (New York, 

 1882), pp. vi, 77. 



218 U.S. Naval Observatory Reports oj the Total Solar Eclipses of July 29, 1878, and 

 January //, 1880 (Washington, D.C., 1880), p. 76. 



95 



