14 OUR PHYSICAL WORLD 



of gravity varies inversely as the square of the distance. So there 

 is a heap of water in the ocean under the moon and one on the 

 opposite side of the earth also. When the earth revolves, the solid 

 land slips along through the water thus held by the moon, the 

 water level along the shore rises, and we say the tide is coming in. 

 As the shore passes out at the other side of the heap the tide falls; 

 so the tide rises and falls twice a day. When the sun assists the 

 moon, as it does when the moon is full or new, the tides are 

 highest. If sun and moon pull against each other, as they do 

 when the moon is at the quarter, the tides are slight. The 



FIG. 5. Diagram of a portion of the earth's path to show the cause of the tides 



amount of the rise or fall of the tide is not great on the open 

 coasts, but when the tide runs up a narrowing bay the rise and 

 fall near the head of the bay may be 60 to 70 feet. 



The friction of the land sliding along under these heaps of 

 water slowly retards the rotation of the earth on its axis and in 

 time must check it. It is supposed that the moon once possessed 

 oceans, and the tides occasioned by the earth's attraction caused 

 its rotation to slow down until now its period of rotation is the 

 same as its time of revolution about the earth, and therefore it 

 keeps the same face always toward us. Its waters have since 

 combined with its mineral materials to form the hydrated miner- 

 als (seep. 58). 



