LETTER II. 



THE TIDES. 



It is not without opposing efforts on the part of 

 the other heavenly bodies that the earth maintains 

 its shape: these efforts indeed are ceaseless, and 

 are put forth most powerfully by the moon by 

 reason of its smaller distance, and by the sun by 

 virtue of its enormous mass. It is true that these 

 forces are unable to draw T the solid parts of the 

 earth straightway from their places ; but they are 

 strong enough to set up some peculiar periodic 

 movements in the readily lifted masses of the air 

 and of the sea. Those of the atmosphere are not 

 easily seen, and are almost masked by other regular 

 and irregular currents. Those of the sea, how- 

 ever, meet our view at once : they consist in an 

 alternate rising and falling of its surface, each 

 recurring regularly twice in every day, and are 

 generally known as the ebb and flow of the water, 

 or, shortly, as the Tides. 



On all sea-coasts, where ^he tides are felt, the 

 waters are found at all times, even when the air is 

 quite calm and clear, to be in restless motion. 

 Great waves are ever rolling in, and breaking on 

 rocky shores into foam and spray. All parts of 



