IV 



THE TIDES 



THE ebb and flow of the tides is altogether too con- 

 spicuous and important a phenomenon of nature to 

 have escaped the notice and observation of the an- 

 cients, and long before Newton's day it was shrewdly sus- 

 pected that some sort of causal relationship subsists be- 

 tween the movements of the moon on the one hand and 

 the behavior of the tides on the other. By the time New- 

 ton arrived on the scene, this suspicion had developed in- 

 to a general scientific conviction, although no explanation 

 had yet been suggested. The circumstantial evidence on 

 which this conviction is founded may be briefly summed 

 up as follows : 



Besides her apparent diurnal revolution around the 

 earth from east to west, the moon has a real motion 

 around our planet in the opposite direction, from west to 

 east, a circuit which she completes (from new moon to 

 new moon) in about 29% days. On account of this latter 

 motion, the moon's time of rising is delayed from one 

 day to another by an average period of 51 minutes. Now, 

 the strange part of it is that high tide arrives at any 

 given port by this same interval of 51 minutes later each 

 successive day, and the natural inference arises that the 

 moon is somehow the cause, and the tide the effect. But 

 the coincidence extends even farther, for it has been 

 demonstrated that the height of the tide varies rhythmi- 

 cally with the moon's changes of phase, being highest at 

 the syzygies and lowest at quadrature. Furthermore, 

 the tide is higher by about 20 per cent when the moon is 

 at perigee than when at apogee. 



