20 POPULAR LECTURES AND ADDRESSES. 



in the channels where there are tide races, gives 

 rise to the generation of heat. Well, now, the end, 

 where it altogether leaves our earth to be dissi- 

 pated through space, is heat. The beginning to 

 which we can at present trace the first source of 

 that energy is in the motions of the moon and the 

 earth. A little consideration shows us, by a very 



ral kind of reasoning, that that particular 

 component of the motion which at zero would 

 rise to no tides must tend to become zero. 

 This we sec as included in a very general proposi- 

 tion applicable to every possible case of action in 

 nature. Now, if the motion of the earth in its 

 rotation, relative to the moon in its revolution 

 round the earth, were zero, there would be no rise 

 and fall of water in lunar tides ; the earth would 

 always turn the same face to the moon, and then 

 it would be always high water towards the moon, 

 low water in the intermediate circle, and high 

 water from the moon, but there would be no 

 motion of the waters relatively to the earth and 



no friction. The tendency of friction must 

 then, according to the general principle, be to 



