575 
figure of the earth . 
longitude of Dover and Falmouth, according to Dr. Tiarks, 
(Phil. Trans. 1824) give by proportion the difference of lon- 
gitude of Dunnose and Beachy Head i° 27' 4", 75. The 
discrepance then is nearly 17"; which (since a small error 
in a or b produces an almost insensible effect on C) implies 
an error of 13" in the sum of the azimuths. This I should 
think is absolutely impossible. We must suppose then either 
that, in consequence of some local attraction, the apparent 
difference of longitudes was altered, or that some error has 
crept into the observations of Dr. Tiarks. The method of 
obtaining apparent time by corresponding altitudes, which 
was employed by Dr. Tiarks, is not the most accurate, nor 
perhaps the most proper, when the whole time in dispute is 
only five seconds. As Dr. Tiarks has not given the details 
of the observations, it is impossible to form any correct 
estimation of their accuracy. If however any local attraction 
have drawn aside the plummet at the eastern station towards 
the east, and that at the western station towards the west, 
the longitude would thus be made to appear smaller than it 
really is. The error produced in the longitude of either 
place would be = co^iat” * an< ^ t ^ lus an error in longitude of 
17" would be accounted for by supposing the plummet at 
Beachy Head drawn 5", 4 to the east, and that at Dunnose 
5", 4 to the west. Each of these is a larger deviation than 
we have reason to believe has taken place at any station in 
England ; and it appears therefore desirable that the obser- 
vations should be re-examined. This operation would not 
be difficult : the easiest method perhaps would be by rocket 
signals; an^ it is obvious, that the local causes which affected 
the difference of longitude found by observations of azimuth, 
