42 6 Capt. Kater’ s experiments for determining the variation 
have been remarked in the Trigonometrical Survey of Great 
Britain. For if the disturbing force in the neighbourhood 
of Arbury Hill, were supposed to be situated to the north of 
that station, the plumb line would be attracted northward, 
the observed latitude would be less than the true, and the 
length of the degree deduced from the arc between Dunnose 
and Arbury would be in excess, and that derived from the 
arc between Clifton and Arbury in defect. This last error 
will be augmented, if we suppose the attraction of the matter 
near Arbury Hill to be felt at Clifton, and the plumb line at that 
station to be drawn towards the south. 
M. Biot, by a comparison of his numerous experiments at 
Unst with those made at Formentara and Dunkirk, in con- 
junction with M. Arago, obtains for the resulting com- 
pression. But if the allowance for the elevation of Formentara 
above the level of the sea, be corrected in the manner sug- 
gested by Dr. Young, the ellipticity should be about • The 
details of M. Biot’s experiments have not yet been published, 
but it affords me much gratification to learn, that the accelera- 
tion of the pendulum between London and Unst, computed by 
M. Biot, from his observations at Unst and those at For- 
mentara, using ylo - f° r compression, differs only o*,6 
from the result of my experiments ; a difference which may 
probably be referred to the superior density of Unst, com- 
pared with that of the substrata of London. 
London, June, 1819. 
