38 Mr. R. Mallet on the Tidal Retardation 



to explain the origination of these currents we must search for 

 other forces. 



In certain cases, if an already existing current accumulates 

 the heated water in greater quantity, the difference of tempera- 

 ture may indeed accelerate this current ; and in the air, if it 

 operates over very considerable spaces, it can generate wind ; 

 but in general it is the quickly condensing aqueous vapour 

 which, by the diminution of its tension, plays the chief part in 

 the production of wind. 



[To be continued.] 



V. Tidal Retardation of the Earth's Rotation. 

 By Robert Mallet, F.R.S.* 



THE general idea of the retardation of the earth's rotation by 

 the great tide-wave acting as a friction-brake as it pro- 

 gresses under the coercion of the moon, commonly ascribed 

 to Mayer, was, I have good reason to believe, anticipated by 

 Emanuel Kant, though I have not been able myself to verify the 

 passage in his writings. Of the real existence of such a re- 

 tarding force, however small may be its effect, there can be little 

 doubt since the masterly researches of Adams upon the moon's 

 acceleration. The subject, probably from its inherent complexity, 

 has attracted but little attention, except from astronomical 

 mathematicians ; and some points respecting it which have been 

 referred to in more popular works, appear involved in some ob- 

 scurity. Professor Tyndall, who, in his ' Heat a Mode of Motion/ 

 gives a very lucid popular account of the phenomena (almost, 

 as he states, in the words of Mayer), has in paragraph 697, 

 p. 483 (4th edit.), the following passage : — " 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 pair of millstones 

 which are 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 heat which lifted the mill-stream to its 

 source." This distinction, it seems to me, cannot be main- 

 tained. The power of a tide-mill is not derived from the 

 rotation of the earth, nor from the retardation of that rotation 

 by the great tide-wave. The sea, no matter from what cause, 

 rises above its normal level, to which it after a time sinks again. 

 If during the interval we can impound a portion of the mass of 

 water so elevated and let it descend through some machine 

 recipient of water-power, we have the tide-mill, the power of 

 * Communicated bv the Author. 



