TIDES. 59 



fluid at any thing like the rapidity of the apparent mo- 

 tion of the sun and moon. If that were the case, both 

 the ocean and the atmosphere would go to those lumi- 

 naries; the gravitation, or difference of gravitation of 

 different parts of the surface to them, is but a very 

 small portion of that toward the earth, and therefore 

 the actual motion of the water or air is really very small, 

 and though upon level and uniform surfaces it be 

 perceptible, the tide or elevation, both of the air and 

 the water, may be produced while there is an actual 

 transfer the other way : there may be tides in the atmos- 

 phere from the east, while the wind, or general motion 

 of the fluid, is strong from the west; and there may be 

 a tide of high water along the course of a river, while 

 the motion of the water in that river is actually down- 

 ward to the sea. The tidal motions of both fluids only 

 tend to put them into circulation, and the interruptions 

 of the land, the different effects of heat, and the varying 

 pressure of the atmosphere upon the surface of the wa- 

 ter, tend to propagate the circulation over the whole 

 surface ; and are, upon the great scale, the means and 

 ministers of life to the whole productions of the globe, 

 in the same manner as the breathing of air and the cir- 

 culation of the proper fluids are to the various species 

 and individuals of which the countless family of all- 

 prolific nature is made up. 



We have mentioned that those causes of circulation 

 have a monthly variation as well as daily motions ; and 

 the reader must have anticipated that they must also 

 have seasonal changes. They would have these, al- 

 though the plane of the orbit in which the moon moves 

 round the earth were the same as that of the earth's 



