iv The Earth a Spinning Ball 5 1 



by very careful measurements of the force of gravity (38, 

 252) by means of a pendulum or fine spring-balance. The 

 weight of a given mass on the Earth's surface depends only 

 on its distance from the centre, and thus as the strength of 

 gravity at different places is found, the figure of the Earth is 

 gradually felt out. The form of the Earth is termed by 

 mathematicians a geoid or earth-like figure ; and it is more 

 accurate to speak of it as a ball than as an ellipsoid or 

 sphere. Yet the difference in shape is so slight that if a 

 geoid or ball, exactly like the Earth, an ellipsoid and a 

 sphere were made each a foot in diameter, it would be 

 quite impossible to tell which was which by the eye or 

 touch. 



84. Structure of the Earth. The Earth is a structure 

 composed of three divisions (i) a vast stony ball termed 

 the lithosphere with an irregular surface, part of which forms 

 the dry land ; (2) a liquid layer resting in the hollows of the 

 lithosphere, a great part of which it covers ; this is termed 

 the hydrosphere or water-shell ; and (3) a complete envelope 

 of gas surrounding the whole to a considerable height and 

 known as the atmosphere or air. 



85. Mass and Density of the Earth. To weigh the 

 Earth, all that is necessary is to measure the attraction 

 of gravity between a large block of metal and a small 

 block set at a measured distance. Then (making allow- 

 ance for^the distance of the small block from the Earth's 

 centre) the attraction of the large block on the small one 

 bears to the weight of the small one, i.e. the attraction 

 of the Earth on it, the same proportion as the mass of the 

 large block bears to the mass of the Earth. Cavendish, 

 who first carried out this experiment a hundred years ago, 

 employed a cumbrous apparatus in which the large attracting 

 mass took the shape of two leaden balls a foot in diameter. 

 The small block consisted of two small leaden balls fixed to 

 the ends of a light rigid rod, which was hung by a fine silver 

 wire. This arrangement is termed a torsion balance, because 

 when the small spheres were attracted by the large ones 

 and moved slightly toward them the wire was slightly 

 twisted, and the force required to twist the wire to that 



