Professor TyndalPs Lectures on Force and on Heat, 533 



of relative motion in it alone. And much more is this true 

 when the tidal motions are contemporaneously acting in all pos- 

 sible directions, some helping and others retarding the mean 

 rotation, and so, if not keeping it constant, at all events redu- 

 cing the retardation to a small residual phenomenon. 



Is then the statement about "the heat produced" and the 

 comparison with the stream-mill an inadvertence ? or do I mis- 

 understand it ? And is the true meaning of the paragraph to be 

 found in the earlier portion, where it is said that "the earth's 

 rotation supplies the loss of that part of the heat which is radi- 

 ated into space V } Of the whole energy extracted from the tide, 

 is a part restored to it in some way by the resulting heat ? and 

 does the rest escape, leaving the earth's rotation in some way to 

 make up the balance ? 



I have already said that the vis viva lost by the earth must be 

 immeasurably small compared to that lost by the tide. But what 

 part of the heat representing the total loss is ultimately radiated 

 into space ? Is it reasonable to suppose it much, if at all, less 

 than the whole ? I should suppose that much the greater part of 

 the heat generated would be at once seized by the water, and in 

 the first instance tend towards increasing the temperature of the 

 ocean, but that this tendency would discharge itself at the sur- 

 face by evaporation and radiation, and that a similar passage 

 through the atmosphere would end in radiating the amount 

 received. And the course of the rest through the solid earth to 

 the atmosphere into space, though more complicated, would, I 

 suppose, be similar. If there is a secular change going on in the 

 temperature of the superficial strata, of the sea, of the air, or if 

 there is any other increasing call for any of the work that heat 

 may do, the case may be otherwise. But all the heat produced 

 and not radiated must be for ever producible in the form of ad- 

 ditional temperature or work, mechanical or <e internal/' done 

 in the system : and I find in the passage quoted neither argu- 

 ment nor even suggestion of any causal connexion cither between 

 the heat retained and the maintenance of the tide, or of the part 

 radiated and the destruction of the rotation. 



I conceive, then, that it is certain that the total heat of the 

 millstones is not "at the expense of," or derived from, the de- 

 struction of the earth's rotation, and that no reason is shown for 

 thinking that the millionth part of it is so. llathcr, I should 

 say that, if there be any "slip" or loss of rotation by the earth, 

 there is ground for thinking the total heat generated is by so 

 much less than it would have been had any extraneous cause 

 maintained the original amount. And this seems to dispose of 

 the main point of the paragraph. 



But further, whether the rotation is diminishing or not, there 



