INTRODUCTION. 11 



tides to more than one-fourtli of the moon's extent. 

 When the power of these luminaries is exerted in 

 the same direction, the result is a higher elevation, 

 called the spring-tide : and, for the reason already- 

 explained, the same occurs when they are in oppo- 

 site quarters of the heavens. On the other hand, 

 when they are in quadrature, that is, when appa- 

 rently separated by just one-fourth of the heavens, 

 the influence of the sun neutralizes, in the ratio of 

 one-fourth, that of the moon ; and hence we have the 

 lowest tides, called neap-tides, soon after the first and 

 third quarters of the moon. 



Local circumstances greatly affect not only the 

 time, but also the height of the tides. In some long 

 bays, which grow gradually narrower, in the form of 

 a funnel, the whole of the increased water which en- 

 tered the mouth of the bay, being confined within 

 A'ery narrow limits, rises rapidly to a great height. 

 Near Chepstow, in the Bristol Channel, for example, 

 the tide rises from 45 to 60 feet, and on one oc- 

 casion, after a strong westerly gale, it even reached 

 to 70 feet. Again, in the Bay of Eundy, in North 

 America, the spring-tides sometimes rise to the 

 astonishing elevation of 120 feet. At the mouths 

 of some large rivers, where the shore is very level to 

 a considerable distance inland, the tide rolls in under 

 the form of one vast wave, which is called the bore : 

 something of this kind occurs in Solway Firth on our 

 own coast ; and it is said that if, when the tide is 

 coming in, a man upon a swift horse were placed 

 at the water's edge, and bidden to ride for his life, 

 the utmost efforts of his steed wo\dd not preserve 



