Proceedings of the Royal Society. 271 
inferred from the trigonometrical survey of Great Britain. It be-~ 
came, therefore, desirable that some further operations should 
be undertaken for the removal or elucidation of this discordance, 
and the following year a similar method was adopted with twenty- 
five chronometers, for determining the difference of longitude be- 
tween Falmouth and Dover; this latter station having been chosen 
as easy of access, and as being perfectly determined ; and the 
computations were made by interpolation, without employing any 
other rates for the chronometers than those which were observed 
in the different trips while they were actually on board of the ship ; 
and latterly, when Dover Roads became unsafe, the operations 
were limited to the distance from Portsmouth to Falmouth: thus 
between the months of July and September the observations were 
made three times at Dover, four times at Falmouth, and three 
times at Portsmouth: and the comparison of their results affords 
a correction of five seconds of time for the difference of longitude 
of Dover and Falmouth, and of three for the difference of Fal- 
mouth and Portsmouth, agreeing completely with the error of four 
seconds attributed, from the observations of the preceding year, 
to the difference of longitude of Falmouth and Greenwich. 
Hence Dr. Tiarks thinks it fair to conclude, that the diameter of 
the parallel circle on which the longitude is measured has in the sur- 
vey been taken somewhat too great, and consequently the earth’s el- 
lipticity greater than the truth. He remarks, that the measurement 
of the spheroidical triangle concerned, determines only the actual 
flatness of the part of the earth’s surface on which it is situated, and 
not the actual magnitude of the whole parallel, unless its curvature 
be supposed perfectly uniform, which we cannot assume with confi- 
dence: while, on the other hand, if we compute the ellipticity 
from the result of the chronometrical determination, it becomes 
si, instead of ;4,, and agrees with the most accurate measure~ 
ments obtained from different principles. The longitude of Fal- 
mouth is finally determined to be 20 minutes, 11.1 seconds of 
time, and that of the British Consul’s garden at Funchal, 1 hour, 
7 minutes, 39 seconds, west of Greenwich. 
U2 
