362 Dr. Tiarks 5 account of 
that between any two successive terms of one set, a term cor- 
responding in time to one of the other set may be determined 
by interpolation, which will represent the difference of the 
chronometer from the mean time of one place at the same 
moment at which the difference from the mean time of the 
other place was determined by actual observation. The dif- 
ference of two such terms of both sets answering to the 
same moment being the difference of longitude of the two 
places, it follows, that the number of results thus obtained, is 
always equal to the sum of the numbers of terms in both 
sets, minus two, or to the number of trips from one place to 
the other, minus one. The chronometers which were em- 
ployed on the occasion were carefully placed on board ship, 
and never removed from their places during the whole time 
of the expedition ; and no rate was therefore used which was 
determined while they were in another situation ; an advan- 
tage, it appears to me, of some consequence. The time was 
determined by numerous equal altitudes of the sun, taken 
with a sextant, for which two assistants carefully noted down 
the moments of observation, by two chronometers ; the differ- 
ences of which from the other chronometers were ascertained 
by several comparisons before and after taking the observa- 
tions on shore. The number of observations being generally 
sufficiently great to destroy, in the mean of them, the small 
errors of each observation, and the rates of the chronometers 
nearly uniform during the time which it was necessary to 
stay on shore, the two chronometers usually gave nearly 
equal results, the mean of which was afterwards adopted in 
deducing the state of the other chronometers. When the 
advanced state of the season rendered it dangerous to ride at 
