286 NEWTON S PEINCIPIA. 



thought he could explain it by a combination of the rotation 

 of the earth about its axis with the annual motion round 

 the sun. The impossibility of deducing an explanation 

 from these premises of the most ordinary phenomena of the 

 tides is evident from what we have said in a previous 

 chapter. Galileo, busy in establishing by new proofs the 

 rotation of the earth, was naturally inclined to find in it 

 the cause of a phenomenon so mysterious, which if it had 

 succeeded would have furnished him with a most powerful 

 argument. Descartes had another equally impossible 

 theory; it was reserved for Newton to suggest the true 

 cause of the motion of the sea. The discoverer of gravi 

 tation could not be long before he saw that whether or not 

 it was the only cause of the tides, it must certainly be one 

 of them. The attraction of the moon could not be the 

 same in all parts of any extended sea. Motion, therefore, 

 must ensue. Nor could any position of rest be ever 

 assumed, because the earth and moon themselves are in 

 motion. And here Newton showed his superiority over 

 those philosophers who afterwards treated of the same 

 subject. He saw that the motion of the tides was a 

 question of Hydrodynamics, and in his First Book he con 

 siders it as such, and has even shown that in one particular 

 case the water would be lowest in that part which is 

 immediately under the moon. That he did not do more is 

 no reproach. Even at the present day the theory of 

 Hydrodynamics is in its infancy : how impossible then it 

 must have been in Newton s time, when the simple laws 

 of the motion of a single particle had only just been 

 understood, to have attempted the consideration of a fluid 

 under the action of complicated forces. Newton gives, 

 therefore, merely a general explanation of the tides, and 

 enters into some numerical calculations merely as a first 

 attempt. 



