42 THEORY OF THE TIDES. 



only show the relative effects of the attractions of the 

 heavenly bodies, irrespective of the innumerable cir- 

 cumstances which modify the rise and fall of the 

 waters on the surface of the globe. 



It is usual to explain the phenomena of the tides 

 by merely taking into consideration what would be the 

 direct action of the heavenly bodies on the mass of 

 the earth and the waters that surround it, supposing 

 the earth to be covered with an universal ocean of an 

 uniform depth ; but this hypothesis omits so many ex- 

 isting circumstances that the theory founded upon it 

 is wholly insufficient for more than the rudest gene- 

 ralization. Properly to understand the action of the 

 tides, all the actual circumstances must be considered. 

 We shall then have, besides the attractions of the 

 masses, to calculate the influence of the movements of 

 the earth and moon as they whirl on their axes, and 

 progress in their orbits ; the form and density of the 

 earth ; the breadth and depth of the oceans and seas 

 on its surface ; the form of continents, and the obstruc- 

 tions offered by iiTegularities in the bed of the sea; 

 not to speak of the laws of the resistance of fluids, and 

 of the motions of waves. Thus, the theory of the tides 

 becomes one of the most abstruse problems of mixed 

 science, and into which I have neither space nor ability 

 to enter. Merely referring the readers therefore to a 

 few works,* in which this subject is treated at large, 

 I shall confine myself to the briefest outline. 



* See the article " Tides and Waves," by Prof. Airy, in Encycl. 

 Metropolitan ; Sir J. W. Lubbock's " Elementary Treatise on Tides," 

 1839; and Johnston's "Physical Atlas, Hydr. Chart. 4." 



