16 THE OCEAN RIVER 



will become a year and that half of the earth would be perma- 

 nently in darkness. 



The second result of the frictional drag is to increase the 

 moon's distance from the earth, at the rate of about five feet 

 in each century. This slowing down of the rotation of the 

 earth and of the revolution of the moon around it and this 

 movement of the moon away from us become of great impor- 

 tance to our ideas of the origin of the earth and its ocean beds 

 when we trace them backward in time. It can first of all be 

 shown that, somewhat earlier than three billion years ago, the 

 moon may have revolved around the earth in a period of only 

 four hours and that the earth itself rotated in exactly the same 

 time. Thus at that distant date there would have been no lunar 

 tides on the earth. Calculations also show that in this far age 

 the moon was only 8,000 miles from the center of the earth, 

 and that a short time before it must have been part of the 

 earth itself, if we accept the findings of some scientists. 



The past now becomes clearer. Somewhere about three or 

 four billion years ago the earth had not given birth to the 

 moon. It was rotating every four hours, and tidal effects were 

 due to the sun alone. This speed of rotation is a particularly 

 dangerous one for a body the size of the earth, according to the 

 popular, but now disputed theory of resonance. A familiar 

 example of resonance is the method we use when we push a 

 child's swing. If the pushes are given out of time with the to- 

 and-fro movements of the swing, the movement will be slowed. 

 If they are given in time with the natural movement of the 

 swing, they are said to be ''in resonance," and the effects of 

 the pushes will be added to each other and will rapidly build 

 up as far as or beyond the safe limit of swinging. In the same 

 manner, if the musical sound of a violin — which is simply 

 a back-and-forth vibration or swinging of air — should have 

 the same frequency of movement as the natural period of 

 a wine glass, the vibrations or oscillations caused in the glass 



