496 THE POPULAR SCIENCE MONTHLY. 



Remember, it is not the mere presence of the tide which produces fric- 

 tion. It is the action of the tide in rising and in falling which accom- 

 plishes the work. If, therefore, the moon moved so that it was always 

 high tide at the same place, the tides could produce no further effect. 

 The spot where the tide is high on the moon is the spot which is 

 toward the earth. It hence follows that the action of the tides will 

 cease when the moon constantly directs the same face to the earth. 

 The moon has thus at length gained a haven of rest from a tidal point 

 of view. No doubt the moon has a high tide and it has a low tide, 

 but those tides no longer ebb and flow : the moon has succumbed to 

 the incessant action of friction, and has assumed the only attitude 

 which can relieve it from incessant disturbance. 



For many centuries it had been an enigma to astronomers why the 

 moon should always turn the same face to the earth. It could be 

 shown that there were many million chances to one in favor of this 

 being due to some physical cause. The ordinary theory of gravitation 

 failed to explain the cause. Every one had noticed this phenomenon. 

 Yet the explanation was never given till lately. It was Helmholtz who 

 showed that this was a consequence of ancient tides, and this simple 

 and most satisfactory explanation had been universally accepted. The 

 constant face of the moon is a living testimony to the power of the 

 tides. What tides have accomplished on the moon is an earnest of 

 what tides will acconrplish on the earth. 



In the great conflict of the tides the earth has already conquered 

 the moon, and forced the moon to render perpetual homage as a token 

 of submission. Remember, however, that the earth is large and the 

 moon is small. Yet, small though the moon is, it gallantly struggles 

 on. " You have forced me," cries the moon to the earth, " to abandon 

 the rotation with which I was originally endowed ; you have com- 

 pelled me to rotate in the manner you have dictated. I will have my 

 revenge. It is true I am weak, but I am unrelenting ; day by day I 

 am exhausting you by the tides with which I make you throb. The 

 time will assuredly come, though it may not be for millions of years, 

 when you shall be forced to make a compromise. When that com- 

 promise is made the turmoils of the tides will cease ; our mutual move- 

 ments will be adjusted. With equal dignity we shall each rotate 

 around the other ; with equal dignity we shall each constantly bend 

 the same face to the other." 



There is another point to be considered. We must not forget that 

 there is a sun in the heavens as well as a moon. The sun also pro- 

 duces tides in the earth. Those tides were much smaller than the 

 lunar tides, so that Ave could afford to neglect them. But we have 

 seen that the lunar tides will gradually decrease to nothing. It be- 

 hooves us then to consider what the solar tides can effect which shall 

 be worthy of our attention. In a lecture which I gave here some 

 years ago, I made allusion to the discovery of the satellites of Mars. 



