FORCE. 377 



shall melt with fervent heat.' The earth's own motion 

 embraces the conditions of fulfilment; stop that mo- 

 tion, and the greater part, if not the whole, of our 

 planet would be reduced to vapour. If the earth fell 

 into the sun, the amount of heat developed by the 

 shock would be equal to that developed by the combus- 

 tion of a mass of solid coal 6,435 times the earth in size. 

 There is one other consideration connected with the 

 permanence of our present terrestrial conditions, which 

 is well worthy of our attention. Standing upon one of 

 the London bridges, we observe the current of the 

 Thames reversed, and the water poured upward twice 

 a-day. The water thus moved rubs against the river's 

 bed, and heat is the consequence of this friction. The 

 heat thus generated is in part radiated into space and 

 lost, as far as the earth is concerned. What supplies 

 this incessant loss? The earth's rotation. Let us look 

 a little more closely at the matter. Imagine the moon 

 fixed, and the earth turning like a wheel from west to 

 east in its diurnal rotation. Suppose a high mountain 

 on the earth's surface approaching the earth's merid- 

 ian; that mountain is, as it were, laid hold of by the 

 moon; it forms a kind of handle by which the earth 

 is pulled more quickly round. But when the meridian 

 is passed the pull of the moon on the mountain would 

 be in the opposite direction, it would tend to diminish 

 the velocity of rotation as much as it previously aug- 

 mented it; thus the action of all fixed bodies on the 

 earth's surface is neutralised. But suppose the moun- 

 tain to lie always to the east of the moon's meridian, 

 the pull then would be always exerted against the 

 earth's rotation, the velocity of which would be dimin- 

 ished in a degree corresponding to the strength of the 

 pull. The tidal wave occupies this position it lies 

 always to the east of the moon's meridian. The waters 



