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CHAPTEE II 



MODERN VIEWS OF THE EARTH* S DENSITY, SHAPE, STRUCTURE, 

 AND ORIGIN. 



THE earth's density, size, and shape, are all closely connected with 

 each other. The earth is five and a half times as heavy as a globe of 

 water of the same size would be. It is twice as heavy as a similar 

 globe of granite, half the weight of a like globe of lead, and about a 

 fourth as dense as though it were made of solid gold. This density 

 of the earth is also called its specific gravity. But specific gravity is 

 the weight of a substance in air divided by the difference between its 

 weight in air and weight in water, thus : 



Weight in air 



= specific gravity. 



Weight in air Weight in water. 



In the case of the earth, the specific gravity is inferred by comparison, 

 not determined by experiment. There are several ways in which the 

 density of the earth has been estimated. The most important of these 

 are known as the Schiehallion method, the Pendulum method, and the 

 method of Cavendish. The Schiehallion method, used by Maskelyne 

 in 1742, is essentially this : If a line with a weight attached to it is 

 suspended, it points towards the centre of the earth. Such a plummet 

 was so placed as to be attracted by the somewhat isolated mountain 

 in Perthshire, named Schiehallion. The size and density of the moun- 

 tain being known from measurement and by weighing samples of the 

 rocks, the amount was calculated by which the line ought to be 

 attracted towards it by the mass of the mountain. And since the line 

 was not drawn towards the mountain nearly so much as it would have 

 been if the earth had been throughout of the same density as the 

 mountain, it follows that the earth as a whole must be much denser ; 

 and, in fact, is found by this evidence to be twice as dense as that 

 mountain, or, in other words, has the density indicated by the num- 

 ber 5 J. This experiment has been repeated at Edinburgh and at Mont 

 Cenis, yielding nearly the same results, though in both these cases the 

 density appeared to be slightly less than at Schiehallion. The Pendu- 

 lum method, invented by Sir G. B. Airy in 1854, consisted in observing 

 the difference between the movements of a pendulum at the bottom 

 of the Harton colliery, near South Shields, and on the earth's surface. 



