. . . Zenith Sector of 19 inches radius to be used when the utmost accuracy 

 was not necessary, and where the transportation of the large one could not 

 be effected without great expense and difficulty. These instruments were 

 principally executed by my late worth and ingenious friend, Mr. Rittenhouse, 

 except some additions which I have made myself. The plumb lines of both 

 Sectors are suspended from a notch above the axis of the instruments in the 

 manner described by the Rev. Dr. Maskelyne, the present Astronomer 

 Royal at Greenwich, in the introduction to the first volume of his Astronom- 

 ical Observations. A particular description of those instruments is rendered 

 unnecessary by being accurately done in a number of scientific works, par- 

 ticularly by M. de Maupertius in his account of the measurement of a degree 

 of the meridian under the Arctic Circle — The Sector is of all mstruments 

 the best calculated for measuring zenith distances which come within its arc. 

 The large one above mentioned [large Zenith Sector] extends to 5 degrees 

 North, and South of the Zenith. Stars when so near the Zenith are insensibly 

 affected by the different refractive powers of the Atmosphere arising from 

 its different degrees of density. Add to this that the error of the visual axis 

 is completely corrected by taking the Zenith distances of the stars with the 

 plane, or face of the instrument both East and West. 



USNM 152079. Figure 80. 



Rittenhouse & Evans, Philadelphia, Pa., 18th century. 



Surveying Compass^ about 1780, made of brass, overall length 

 13% in., diameter of dial 5)1 in., silvered bubble level, vernier on 

 alidade. The glazed dial, engraved "Rittenhouse & Evans," is 

 fitted with a brass cover. 



This instrument was made during a brief partnership between 

 David Rittenhouse and David Evans, a clock- and watchmaker of 

 Philadelphia and Baltimore. It was one of several owned and 

 used by John Johnson in 1818 for surveying the boundaries 

 between Canada and Maine. 



The survey, made in compliance with the Treaty of Ghent, is 

 described in The Collections of the Maine Historical Society (Port- 

 land: Hoyt, Fogg & Donham, 1881, vol. 8, p. 20): 



Thomas Barclay, of whom we have heard more than once before, as a 

 Commissioner under the treaty, on the part of Great Britain, and Cornelius P. 

 Van Ness, on the part of the United States, were appointed Commissioners to 

 ascertain and run the line. An actual survey was arranged, and surveyors 

 appointed, to wit: Charles Turner, Jr., on the part of the United States, and 

 Colin Campbell on the part of Great Britain. About twenty miles of the 

 line was surveyed, then the work was discontinued, never to be resumed; but 

 an exploring survey was commenced by Colonel Bouchette, on the part of 

 Great Britain, and John Johnson, on the part of the United States. These 

 gentlemen made an exploring line in 1817, extending ninety-nine miles from 

 the monument at the head of the river St. Croix, and made separate reports of 

 their doings. In 1818 Mr. Johnson, with Mr. Odell, who had taken the place 



146 



