FORCE. 377 



meter. It has been prophesied that < the elements 

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

 embraces the conditions of fulfilment ; stop that motion, 

 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 combustion of a mass 

 of solid coal 6435 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 meridian ; 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 augmented it ; thus the action of all 

 fixed bodies on the earth's surface is neutralised. But 

 suppose the mountain 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 diminished in a degree corresponding to the 

 strength of the pull. The tided wave occupies this 



