i 4 EVOLUTION OF THE EARTH 



force of its revolution. Suppose, in other examples of 

 heavenly bodies, the tidal pull to be many times stronger, the 

 self-gravitative cohesion to be many times weaker. A limit 

 will be reached at which the body may be pulled to pieces. This 

 phenomenon, which has been actually observed in the case of 

 comets passing close to the sun, has been called tidal dis- 

 ruption. 



FIG. 1. Diagram to illustrate tidal forces. 



Let M and N in Figure i be two bodies passing each other 

 in space, and consider the action of the larger on the smaller. 

 According to Newton's law, the bodies attract each other 

 directly as their masses and inversely as the square of their 

 distances, causing them to swing toward each other while 

 passing by, but soon losing influence as they separate in their 

 journeys through space. Consider three particles, a, b, c, on 

 the line of attraction, taking them as separate parts of the 

 smaller body. But a is nearer to M than is b, and b is nearer 

 than c. Therefore if we represent the relative attractions by 

 lines, these lines will correspond to the distances which the 

 particles would move in a given time if free to obey the attrac- 



