PHYSICAL GEOGRAPHY. 



Disturbances of equilibrium, and consequent movements 

 af the waters, are of three kinds : 1st, irregular and transitory, 

 occasioned by winds, and producing waves which sometimes 

 during storms, and at a distance from coasts, attain a height 

 of more than 37 English feet ; 2d, regular and periodic, 

 dependent on the position and attraction of the Sun and 

 Moon ; and, 3d, the less considerable but permanent phce- 

 nomena of oceanic currents. The tides extend over all 

 seas (except those small inland seas where the ebb and 

 flow is scarcely if at all perceptible) ; they have received a 

 complete explanation by the Newtonian doctrine, and have 

 been thus brought " within the domain of necessary facts." 

 The duration of each of these periodical oscillations ot 

 the sea is rather more than half a day; their height 

 in the open ocean is not more than a few feet ; but when 

 the configurption of a coast opposes the progress of the 

 wave, they may reach upwards of 50 feet, as at St. Malo, 

 or even of 70 feet, as in the Bay of Pundy. The great 

 geometer, Laplace, has shewn that, regarding the depth of the 

 sea as inconsiderable in relation to the semi-diameter of the 

 earth, the stability of equilibrium of the ocean requires 

 that the density of its fluid mass should be less than the 

 mean density of the earth. We have already seen that the 

 mean density of the earth is actually five times that of water. 

 Tides, therefore, caused by the action of the Sun and Moon 

 can never overflow the elevated portions of the dry land; nor 

 can they have transported the remains of marine animals to 

 the summits of the mountains where they are now found. ( 367 ) 

 It is no small testimony of the value of analysis, sometimes 

 so contemptuously regarded in the unscientific circles of 



