in the Air and in the Sea, 107 



raising of the level will be less when the major axes of the two 

 ellipsoids are perpendicular to each other — that is, at the times 

 of the first and last quarters of the moon. 



All this we see confirmed in nature by the phenomena of ebb 

 and flow of the tides. Many renowned mathematicians (among 

 whom Newton, Euler, Laplace, and Airy occupy the first place) 

 have endeavoured to determine by very ingenious mathematical 

 calculations the laws of the tides ; their theories, however, do not 

 in all respects perfectly agree with the phenomena. We find, 

 for instance, that on the coasts of the islands in mid-ocean the 

 tide often rises only a few inches, and seldom amounts to more 

 than from 2 to 3 feet, while one would think that it was just in 

 the open ocean that the tide could be fully developed. Accord- 

 ing to the theory, the tide should assume the greatest dimen- 

 sions in the tropical regions — instead of which, we find that, 

 with very trifling exceptions, it is very moderate in the tropics, 

 and does not reach, by a long way, the height it attains in the 

 English Channel or on the coasts of the Bay of Fundy in Nova 

 Scotia. Airy based his tide-theory on the theory of waves, and 

 hence ascribes to the water-particles only a vertical oscillating 

 motion. But, self-evidently, there cannot be anywhere an ele- 

 vation of the sea-face, unless the necessary water flows to the 

 place of elevation, water being incapable of elastic expansion ; 

 hence, among tidal phenomena, the existence of a horizontal 

 motion of the water is undeniable. Nay, the horizontal motion 

 must be very considerable, since it is able, in the course of a 

 few hours, to call forth a not unimportant elevation of the 

 water- surface over many thousands of square miles. 



If the earth stood still and the same points had the sun or 

 the moon in the zenith constantly, the surface of the sea would 

 probably take the position of the tidal ellipsoid given by the 

 theory, and always retain the same form. But now the relative 

 position of the sun and moon to the earth is perpetually chang- 

 ing through the rotation of the latter ; and therefore a very 

 large volume of water and air must continually flow out of one 

 part of the ocean into the other, in order to compensate the far- 

 extended disturbance of equilibrium. 



Now, as the relative change of place of the sun and the moon 

 i s very rapid, while for the complete formation of the tidal ellip- 

 soid a certain time is necessary, it may be that the ellipsoid has 

 not sufficient time to take its perfect form ; the tendency, how- 

 ever, to form it must call forth currents in air and water, which 

 will constantly follow the motions of the moon and the sun. If 

 this be admitted, it explains to us why, in the open ocean, where 

 these currents proceed undisturbed, no tide, or a very slight one, 

 is observed ; for only where insufficient depth or the shape of 



