﻿of Tidal Action. 217 



the amount of heat generated by these oscillatory movements ; 

 and more exact investigations show that the changes which take 

 place under such circumstances would ultimately cause these 

 second-rate planets to occupy the same time in their rotation 

 and in their orbital revolution, while the axis of each would 

 become nearly, if not exactly, perpendicular to the plane of its 

 orbit. On the surfaces of bodies having their motions so ad- 

 justed in the course of time, gravity, though not uniform in dif- 

 ferent localities, would be exempted from any material periodical 

 changes, and the repose of the planetary structure could not be 

 seriously affected by the central disturbance. From the evidence 

 of the telescope, it would appear that this peculiar arrangement 

 for keeping the same side of a moon always turned to its primary 

 invariably prevails in all secondary systems of celestial bodies ; 

 and it may justly claim much interest from astronomers, whether 

 it be regarded as a beneficent provision for preventing excessive 

 tides, or as the result of tidal action during past ages. 



Though the great disturbance to which satellites are exposed 

 in narrow orbits, and which, in the absence of certain conditions, 

 is capable of producing commotions even in the most stubborn 

 solid matter of which they maybe composed, has been the main 

 object of my researches in this department of science, I think it 

 advisable to introduce a few items in regard to the tides on our 

 own globe. In a paper published in the twenty-seventh volume 

 of the Reports of the British Association for the Advancement 

 of Science, I gave a popular exposition of the mode in which the 

 vast tidal wave on our oceans must permanently alter the mo- 

 tions of the earth and the moon. I have since learned that the 

 views I then presented respecting the loss of terrestrial motion 

 were not entirely new ; and as I understand that the subject is 

 already receiving much attention from some eminent astrono- 

 mers, I do not think it proper to treat on it at a greater length 

 than the main object of my researches demand. I shall accord- 

 ingly take up only the more simple cases of this kind of action, 

 as they will be sufficient to show bow the secular changes with 

 which the tides on our oceans must be attended are divided be- 

 tween our earth and its satellite. 



For the source of the slow changes in question, we may first 

 look to the effects of the attraction between the moon and the 

 portion of water which swells, by her influence, above the mean 

 level of the ocean. The presence of so large an amount of pro- 

 tuberant matter on opposite sides of the terrestrial spheroid must 

 produce a slight deviation in the direction of gravity between the 

 earth and the moon, and thus lead to permanent changes in their 

 movements. To determine the relative rates at which motion is 

 slowly lost to each body from this cause, and to show how far one 



