WHAT DID THE ANCIENTS KNOW OF TIDES ? 37 



attracts the ocean, and thus tides arise in the larger seas. If 

 the earth ceased to attract the waters, they would rise and flow 

 up to the moon." 



The general notion of a mutual attraction, however, did no 

 more than point out the way for the solution of the problem, 

 and it was reserved to our great Newton to accomplish the 

 prophecy of his great predecessor, "that the discovery of the true 

 laws of gravitation would be accomplished in a future generation, 

 when it should please the Almighty Creator of nature to reveal 

 her mysteries to man." 



Newton was the first who proved that the tide-generating 

 power of a celestial body arises from the difference of the at- 

 traction it exerts on the centre and the surface of the earth. 

 Thus it was at once made clear how the water not only rises on 

 the surface facing the moon, but also on the opposite side of the 

 earth, as in the latter case the moon acts more strongly on the 

 mass of the earth than on the waters which cover the hemisphere 

 most distant from her. The evident consequence is that the 

 earth sinks (so to say), on the surface turned from the moon, 

 whereby a deepening of the waters, or, in other words, a rising of 

 the tide, is occasioned. 



It now also became clear how the moon, whose attractive 

 power upon the earth is 160 times smaller than that of the 

 sun, is yet able to occasion a stronger tide, since, from her 

 proximity to the earth, she attracts the surface more forcibly 

 than the centre with the thirtieth part of her power, while the 

 distant sun occasions a difference of attraction on these two 

 points equal only to one twelve-thousandth part of her attrac- 

 tive force. 



Now also a full explanation was first given why the highest 

 tides take place at new and full moon : that is, when the moon 

 stands between the sun and the earth ; or the latter between the 

 sun and the moon ; as then the two celestial bodies unite their 

 powers; while at half-moon the solar tide corresponding with 

 the lunar ebb, or the lunar tide with the solar ebb, counteract 

 each other. 



But even Newton explained the true theory of the tides only 

 in its more prominent and general features, and the labours of 

 other mathematicians, such as MacLaurin, Bernoulli, Euler, 

 La Place, and Whewell, were required for its further development, 



