212 SEA AND LAND 



all the orbs of space exercise a measure of attraction on our 

 own sphere, the moon and sun, owing to nearness and mass 

 respectively, are the only bodies which effect a recognizable 

 influence upon its surface. These two bodies pull so strongly 

 upon this planet that in the widest and deepest ocean they 

 form two tidal waves opposite each other, each some thousands 

 of miles wide and a foot or two in height. If the whole surface 

 of the earth were occupied by the sea waters everywhere to 

 the depth of four miles, say, the maximum height of the tide 

 w^ould probably be about a foot. Although this tide would 

 move around the earth beneath the equator with the speed 

 of a thousand miles an hour, it could exercise no considerable 

 effect on the conditions of the sphere, for, according to the 

 supposition, it would meet no shores or shallows ; the whole 

 consequences of the tidal movement would be the application 

 of a slight friction to the bottom, which, being applied against 

 the direction of the earth's rotation, would tend very slowly 

 to overcome the momentum with which this sphere turns 

 upon its axis. 



In the existinij condition of the earth the lunar and solar 

 tides, though of slight altitude in the wider parts of the great 

 southern sea and Pacific Ocean, become wedged in between 

 the convercfincf shores of the lesser water basins in such a 

 manner that the water may be lifted in each tidal interval t(3 

 much greater heights. Very commonly this increase in alti- 

 tude amounts to ten or twelve feet, and in exceptional cases to 

 fifty feet or more. As the energy which a tidal wave applies 

 to a coast-line is in a general way proportional to the rise 

 and fall of the undulation, it is easy to see that the share 

 of this impulse derived from the attraction of the sun and 

 moon, exerted on different parts of the shore, is exceedingly 



