290 Mr. James Croll on the Influence of the 



transferred to the solid particles of the earth against which they 

 rubbed, then the earth, as a whole, would not have lost any of 

 its vis viva of rotation. But the motion of the molecules is 

 transformed into heat- vibrations, and thus lost in so far as rota- 

 tion is concerned. 



It will be observed that at the time when the molecules of the 

 water are losing their motion, they are moving round both the 

 centre and the centre 0'. The question now suggests itself, 

 whether is the heat produced at the expense of the motion round 

 the centre O, or round the centre 0'? If we assume that it is 

 at the expense of the latter, that is to say, at the expense of the 

 earth's motion round the common centre of gravity of the earth 

 and moon, then we must conclude that the velocity of the earth's 

 motion round this centre is diminishing ; and if diminishing, 

 then the earth must be falling towards this centre ; or, in other 

 words, falling towards the moon. And if the earth and moon 

 are approaching toward each other, then the velocity of their 

 motion round this centre must be increasing. But to assume 

 that the friction of the waters in this case could have any influ- 

 ence on the general motion of the earth and moon round their 

 common centre, seems to be in opposition to the principle that 

 the general motion of a system cannot be affected by the relative 

 motion of its parts. Hence we infer that the entire heat pro- 

 duced in the present case is derived from the motion round the 

 centre ; in other words, from the diurnal rotation of the earth. 



It will be observed that the way in which we have explained 

 how the heat is produced, and consequently the earth's diurnal 

 motion destroyed, is entirely different from that pointed out by 

 Mayer. According to his view, when the wave passes the meri- 

 dian or point B, the moon, as it were, takes hold of it and pulls 

 it back ; the solid part of the earth at the same time moving 

 forward under the wave, rubs against its bottom and thus gene- 

 rates heat. This rubbing process acts as a drag upon the rota- 

 tion of the solid earth and thus consumes its motion. In this 

 case it is the solid part of the earth that loses motion by being 

 held back by the molecules of the water. In the case to which 

 we have directed attention, the reverse takes place. It is the 

 molecules of the water that lose their motion by being held back 

 by the solid part of the earth. But the general effect on the 

 earth's rotation is the same in both cases. For, in so far as the 

 globe is concerned, it is the same whether it be the sea or the 

 dry land which in the first instance loses its motion. 



M. Mayer is perfectly correct in his explanation ; but then 

 he only exhibits one part of the process. It will be observed 

 that the heat is produced, according to his explanation, when 

 the solid parts of the earth are endeavouring to move faster than 



