in the length of the pendulum vibrating seconds. 417 
Of the Figure of the Earth. 
The deviation of the figure of the earth from a perfect 
sphere, is expressed by a fraction, having for its numerator 
the difference between the equatorial and polar diameters, 
and for its denominator the diameter at the equator ; this is 
termed the compression or ellipticity. 
If the earth were a perfect sphere, composed of homogene- 
ous materials, as a fluid, and at rest, gravity at every point in 
its surface would be the same. But if this sphere were made 
to revolve about an axis, its particles would endeavour to fly 
off with a centrifugal force proportionate to the distance from 
the axis of rotation ; the equatorial parts would become ele- 
vated, those at the pole and its vicinity depressed, and the 
sphere would assume the form of a spheroid, the centrifugal 
force thus generated acting in opposition to gravity, and 
diminishing it more and more from the Pole, where the cen- 
trifugal force is nothing, to the Equator where it is a 
maximum. 
But besides this diminution of gravity from centrifugal 
force, in proceeding from the pole to the equator, a farther 
reduction takes place in consequence of the elliptical form 
which the earth has now assumed. For the parts about 
the Pole being nearer to the centre of the spheroid than those 
at the Equator, will be more strongly attracted, and this farther 
reduction of gravity, whatever it may be, varies with the 
figure of the earth, and as we shall presently see, with a 
variation in the density of the strata of which it is composed. 
If we conceive two fluid columns meeting in the centre of 
such a spheroid, the one proceeding from the Pole and the 
mdcccxix. 3 I 
