ORIGIN OF THE FIGURE OF THE EARTH. n 



polar protuberances, and transport the materials towards the equator, 

 owing to the action of centrifugal force. At present the polar diameter 

 of the earth is nearly 26 J miles less than its equatorial diameter. There 

 is no reason, however, for supposing that the earth was ever more 

 nearly spherical than now, while, on the contrary, the shape may not 

 improbably have been approximating towards the sphere from a remote 

 period. The flattening of the earth's poles which gives it the orange- 

 like form called an oblate spheroid, is so small in amount that the eye 

 is quite unable to detect the flattening upon any accurate model of a 

 globe that can be made. This figure may result from many causes. 

 Seeing that the excess of length of the equatorial over the polar 

 radius amounts to but little more than the difference between the 

 greatest mountain height -and greatest ocean depth, that fact is con- 

 clusive proof that the earth is sufficiently elastic to owe its flattened 

 form to rotation alone, especially when it is remembered that the 

 centrifugal force or tendency of things to fly from the earth is at 

 the equator o-J-g-th of the force of gravity which draws all things 

 towards the earth's centre of attraction. Hence the shape of the earth 

 may be due entirely to deformation, or alteration from the spherical 

 form, consequent upon rotation, so that altered position of the earth's 

 axis would explain emergence and submersion of land. If any part 

 of the interior of the earth should be fluid, it is possible that the way 

 in which centrifugal force might influence the tendency of internal 

 heated matter to fly towards the equatorial region, might appreciably 

 affect the expansion of the rocks in the equatorial plane, and thus 

 account for a difference between polar and equatorial diameters. Sir 

 William Thomson has stated that if the earth were wholly composed 

 of glass, its mean expansion for every degree increase of Fahrenheit 

 temperature between 30^ and 212 would be i part in 69,660; if it 

 were of iron the expansion in the same limits would be i part in 

 50,760; while if the earth were all copper, the expansion for each 

 degree would be i part in 34,920. If, for the sake of illustration, 

 we assume the mean temperature of the earth at the equator to be 

 80, and suppose no increase of temperature to take place towards its 

 centre, and further suppose the earth to consist of copper, then there 

 would be an increase of the earth's diameter in the equatorial region 

 of many miles, owing to the expansion of the metal ; and if no corre- 

 sponding or any less elongation took place towards the poles, there 

 would be a considerable equatorial bulge due to this cause. It is 

 therefore probable that the earth's internal heat has to be considered 

 as a factor which has influenced its form. 



The Earth's Solidity inferred from the Stability of its Figure. 

 The difference between the polar and equatorial diameters being -g^th 

 of the earth's diameter, gives the earth's surface a curve which deviates 

 from the circle towards an ellipse by about i in 300. And this 

 fact coupled with some remarkable discoveries in the relative motions 

 of the moon and earth made by Professors Adams and Delaunay in 

 1859 and 1866, led Professor Sir William Thomson to speculate from 

 the earth's form upon the length of time for which the earth may 



