Page:Popular Astronomy - Airy - 1881.djvu/256

242 force" has been used to express the tendencies of the various parts of that hoop to acquire greater distances from the central bar. It is a bad term, because in reality there is no force. Perhaps it would be better to say "centrifugal tendency," tendency to recede from the centre, which will in all cases require a force to control it. Now this centrifugal tendency tends to change the figure of the earth; but the consideration of the centrifugal tendency alone is not sufficient to give us the means of calculating what the form of the earth will be.

Newton was the first person who made a calculation of the figure of the earth, on the theory of gravitation. He took the following supposition as the only one on which his theory could be applied: he assumed the earth to be a fluid, or at least to be so far fluid in all parts below the surface, that its form would be the same as if it were entirely fluid. This fluid matter he assumed to be equally dense in every part, so that it was composed of no heavier matter at the centre than at the circumference. For trial of his theory, he supposed the fluid earth to be a spheroid; he then computed the attraction of the whole spheroid upon every one of its component particles of fluid; with this he combined the centrifugal tendency; and then he examined whether, by giving a proper degree of ellipticity to the assumed spheroid, the forces computed on this supposition would be such as would keep the fluid in the spheroidal form which he had supposed to be the earth's form. Now upon the theory of gravitation it is evident that the attraction of a sphere is not the same thing as the attraction of a spheroid. It is necessary to compute what the attraction of this spheroid is, before we can enter into the effect of its