Bond, Graham, and Peirce, on the Latitude of Cambridge. 185 
tude accurate to the tenth of a second, and thereby resolve some 
interesting inquiries in topography and geology. 
Great changes of temperature occurred in the course of the ob- 
servations, which materially affected the level, and thereby most 
seriously interfered with the efforts to attain accurate results. The 
observers were, moreover, wholly unused to this class of observa- 
tions, neither of them having before observed upon the prime 
vertical. 
DeEscRIPTION OF THE INSTRUMENTS. 
Chronometer. — The time was kept, throughout the observations, 
by a chronometer belonging to Major Graham, and numbered 2419. 
This watch had been proved, by previous use, to be of uncom- 
mon excellence. Its rate, determined by means of the meridian 
transit instrument, was small, and is shown in Table I. 
Transit Instrument.—The transit instrament, which was em- 
ployed in the observations upon the prime vertical, was made by 
Troughton and Simms, of London, and was kindly lent to the Ob- 
servatory, for this purpose, by Major Graham. It is of about four 
feet focal length, and three inches aperture, with very clear and 
distinct vision. It has seven vertical and three horizontal wires; 
the seven vertical wires are described in the observations as .4, B, 
C, D, E, F, G; the order in which they are lettered commences 
with the wire nearest to the illuminated end of the axis. The in- 
tervals between the wires were not known from any independent 
astronomical observations ; but this is unimportant, for a knowledge 
of them is not required in the methods of observation and re- 
duction here employed. These intervals have, however, been ap- 
proximately determined by terrestrial observations and the microm- 
eter; and a comparison of them with their values deduced from 
34 
