372 PHYSIOGRAPHY. [chap. 



in regular orbits around the sun. By far the greater number 

 of these bodies are comparatively small and unimportant ; 

 but eight of them are large planets, of which the earth is 

 one, though by no means the largest. All these planets 

 are retained in their orbits by the force of gravitation 

 exerted by the sun, which forms the great centre of the solar 

 system. 



Let a ball be tied to a piece of string, and whirled rapidly 

 round ; and while revolving in this way, let the string be 

 suddenly cut. The ball does not cont'nue to move in a 

 circle, but darts off, in a straight course, until brought to the 

 ground by terrestrial gravity. In like manner, the earth 

 would rush into space in a straight path, if the bond of 

 gravitation, the invisible string, were broken between the sun 

 and the earth. The revolution of the earth in a nearly 

 circular orbit is, therefore, maintained by means of gravita- 

 tion. But gravitation is a force which varies in intensity 

 according to the distance, in such wise that, if the distance 

 be doubled, the intensity is diminished to one-fourth, if 

 trebled, it is diminished to one-ninth, and so forth. It was 

 shown at p. 354, that the earth is nearer to the sun at one 

 time of year than at another ; hence the attraction between 

 the sun and the earth will vary at different seasons. In fact, 

 when the earth is at perihelion, the attraction is greater, and 

 the rate of revolution consequently greater also, than when 

 she is at aphelion. This want of uniformity in the rate at 

 which the earth travels is sufficient to explain the statement 

 at p. 342, that a solar day is not always of the same length. 

 Hence the introduction of the mean solar day of civil time. 



Everything upon the earth's surface is subject to terres- 

 trial gravity. Every particle of water tends to fall towards 

 the centre of the earth, and thus the waters of the ocean 

 are bound down so as to form an envelope around the 



