the mean Denjity of the 'Earth. 779 
In order now to compare this attraction with that of 
the whole earth, this body may be confidered as a fphere, 
and the obfervatories as placed at its furface ; face the very 
fmall differences of thefe fuppofitions from the truth, are 
of no confequence at all in this comparifon. Now the at- 
traction of a fphere, on a body at its furface, is known to 
be = jcdf where d is = the diameter of the fphere, and c — 
3 • 1 4 1 6 = the circumference of the circle of which the dia- 
meter is 1 . But cd is = the circumference of the circle to 
the diameter d; and therefore the attraction of a fphere will 
be expreffed by barely j- of its circumference ; which is a 
theorem well adapted to the computation in hand. The 
length of a degree in the mean latitude of 45 0 , is 57028 
French toifes (fee p. 327. Phil. Tranf. 1768): and the 
fame refult nearly is obtained by taking a mean among 
all the meafures of degrees there put down, that mean 
being 57038 toifes. I lhall therefore ufe the round 
number 57030 as probably nearer the truth. This 
number being multiplied by 6, the product 342180 
fhews the number of French feet in one degree; but, 
by p. 326. of the fame volume, the lengths of the Paris 
and London feet are as 76^734 to 72, that is, as 4^263 
to 4; therefore, as 4 : 4^263 : : 342180 : 364678 = the 
Englifh feet in one degree ; and this being multiplied by 
360 the whole number of degrees, there refults 
5 D 2 131284080 
