THEIR CAUSE. 23 



still greater rise of the waters — the storm-tides. 

 On the coasts of Germany and the Netherlands 

 they sometimes rise to twice the height of the 

 common spring-tides. 



The connection between the tides, and the 

 place of the moon and of the sun, was known to 

 the ancients ; but it was Newton who first threw 

 a clear light on this fact. His theory has since 

 been so fully established, chiefly by the calculations 

 of Laplace, founded on a great number of obser- 

 vations made on the west coast of Prance, that, 

 except when foreign influences, such as storms, 

 chance to interfere, not only the times, but also 

 the inequalities in height, of the tides which will 

 occur at any place, may be known by calculation 

 beforehand. 



The phenomenon of the tide is caused by the 

 mutual attraction of the earth and the moon, and, 

 though in a less degree, of the earth and the sun. 

 Let us first, to simplify the explanation of this, 

 consider one of these agencies by itself, and that 

 on the supposition that the surface of the earth 

 is formed by one entire ocean. The attraction 

 exerted by one heavenly body upon the particles 

 of another, is the less, as you know, the farther the 

 two are apart. For this reason, the moon, during 

 its apparent daily journey round the earth, always 

 most strongly attracts those parts of the latter 

 which lie exactly opposite and nearest to itself, 



