DIMENSIONS. 



according to the computations of the most eminent and recent 

 authorities : 



The close coincidence of these results supplies a striking example 

 of the precision to which such calculations have been brought. 



37. The departure of the terrestrial spheroid from the form of an 

 exact globe is so inconsiderable that, if an exact model of it 

 turned in ivory were placed before us, we could not, either by 

 sight or touch, distinguish it from a perfect billiard ball. A 

 figure of a meridian actually drawn on paper could only be 

 distinguished from a circle by the most precise measurement. 



38. The magnitude of the earth being known with great preci- 

 sion, the determination of its mass and that of its mean density 

 become one and the same problem, since the comparison of its 

 mass with its magnitude will give its mean density, and the 

 comparison of its mean density with its magnitude will give its 

 mass. 



The methods of ascertaining the mass or actual quantity of 

 matter contained in the earth are all based upon a comparison of 

 the gravitating force or attraction which the earth exerts upon an 

 object with the attraction which some other body, whose mass is 

 exactly known, exerts on the same object. It is assumed, as a 

 postulate or axiom in physics, that two masses of matter which at 

 equal distances exert equal attractions on the same body, must be 

 equal. But as it is not always possible to bring the attracting and 

 attracted bodies to equal distances, their attractions at unequal 

 distances may be observed, and the attractions which they would 

 exert at equal distances may be thence inferred by the general law 

 of gravitation, by which the attraction exerted by the same body 

 increases as the square of the distance from it is diminished. 



39. To solve this celebrated problem, it is necessary to bring 

 the whole mass of the globe into direct comparison with some 

 object whose mass is exactly known. This was accomplished first 

 by Dr. Maskelyne, and afterwards by Cavendish. The former 

 compared the attraction of the earth with that of a mountain in 

 Perthshire, called Schehallion; the latter compared it with the 

 attraction of a large ball of metal. Both obtained nearly the 



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