C 531 ] 



LXX. Observations on a passage in Professor Tyndall's Lectures 

 on Force and on Heat, By J). J). Heath, Esq* 



IN the abstract of Professor Tyndall's Lecture on Force deli- 

 vered at the Royal Institution June 6, 186.2, there occurs 

 the following passage : — 



<e There is one other consideration connected with the perma- 

 nence of our present terrestrial conditions, which is well worthy 

 of our attention. Standing upon one of the London bridges, we 

 observe the current of the Thames reversed, and the water poured 

 upward twice a day. The water thus moved rubs against the 

 river's bed and sides, and heat is the consequence of this fric- 

 tion. The heat thus generated is in part radiated into space, 

 and then lost, as far as the earth is concerned. What is it that 

 supplies this incessant loss ? The earth's rotation. Let us look 

 a little more closely at the matter. Imagine the moon fixed, and 

 the earth turning like a wheel from west to east in its diurnal 

 rotation. Suppose a high mountain on the earth's surface; on 

 approaching the moon's meridian, that mountain is, as it were, 

 laid hold of by the moon, and forms a kind of handle by which 

 the earth is pulled more quickly round. But when the meridian 

 is passed the pull of the moon on the mountain would be in the 

 opposite direction, it now tends to diminish the velocity of rota- 

 tion as much as it previously augmented it ; and thus the action 

 of all fixed bodies on the earth's surface is neutralized. But sup- 

 pose the mountain to lie always to the east of the moon's meri- 

 dian, the pull then would be always exerted against the earth's 

 rotation, the velocity of which would be diminished in a degree 

 corresponding to the strength of the pull. The tidal wave occu* 

 pies this position : it lies always to the east of the moon's meri- 

 dian; and thus the waters of the ocean are in part dragged as a, 

 brake along the surface of the earth, and as a brake they must 

 diminish the velocity of the earth's rotation. The diminution, 

 though inevitable, is, however, too small to make itself felt 

 within the period over which observations on the subject extend. 

 Supposing then that we turn a mill by the action of the tide, 

 and produce heat by the friction of the millstones ; that heat 

 has an origin totally different from the heat produced by another 

 mill which is turned by a mountain stream. The former is pro- 

 duced at the expense of the earth's rotation, the latter at the 

 expense of the sun's radiation," 



The paragraph is repeated word for word in the ' Lectures on 

 Heat.' There is a reference to Mayer in both places, which 

 leaves me in doubt whether Professor Tyndail has given any 

 serious attention to the matter himself, or has merely copied 



* Communicated by the Author. 



