53-i Mr. D. D. Heath on a passage in 



does not appear to me to be auy cogency in the very picturesque 

 argument or illustration by which the affirmative is enforced. 



For what are the essential conditions of the " mountain on the 

 earth's surface? " It is fixed to the solid earth, otherwise equably 

 rotating and indifferent to the moon's action. The direct action 

 is on the mountain alone, which therefore begins to move. This 

 motion is arrested by the fixed attachment of the base, and there- 

 upon a tension is set up which is the efficient cause of the rest of 

 the mass participating in the impressed motion. The moving 

 force is the attraction on the mountain, and the mass moved the 

 whole earth, and the numerical calculation is perfectly definite : 

 but all depends on the actual strain. If the mountain were a 

 satellite close to the surface and subject to the attraction of the 

 solid earth, it would itself be affected by the moon's attraction, 

 but would not affect the earth. Now none of these conditions 

 are proved to be present, and some are absent, in the tidal phe- 

 nomena. There is no more cohesion between the crest of the 

 wave and its base than between an apparently fixed cloud and 

 the mountain top on which it rests : both are merely permanent 

 shapes of shifting materials. If one were to try to form the 

 equation for the moving force and the mass moved, how would 

 one proceed? The only connexion is that of friction and of 

 pressure. Both of these (if the latter can be at all conceived as 

 an unilaterally effective force exerted by the superimposed crest 

 on the subjacent waters) depend directly on the velocities and in- 

 ternal disposition of the waters ; only indirectly, through these, 

 on the moon's action ; and for aught that is shown these may 

 be acting in the direction of the rotation at that point, and at 

 any rate are as likely as not to be less energetic there, in whatever 

 direction, than at some other points. All that can be said, as it 

 appears to me, is that the moon's attraction at the crest tends to 

 increase any existing velocity towards the west, or to diminish 

 any in the opposite direction. But then the rest of the ocean is 

 swaying to and fro also, and it cannot be asserted that it (like 

 the supposed solid globe) is indifferent, or that this increased 

 attraction at the crest may not be merely the exact equipoise 

 wanted to prevent an acceleration of rotation. 



The question, therefore, whether "the waters are in part 

 dragged as a brake from east to west," seems to me unaffected, 

 or at least not settled by the fact of the existence of crests lying 

 obliquely to the moon. Are they ? Laplace and Airy say they 

 are not*; but in the cursory inspection I have made of their pages 

 I have not lighted upon the proof of their assertion. They also 

 give proofsf that precession and nutation (and a fortiori the 



* Systeme du Monde, p. 26/. Enc. Metr. "Tides and Waves," p. 126. 

 t Mecanique Celeste, part 1, book v. sects. 11, 12. Enc. Metr. qu. sup. 



