292 Mr. James Croll on the Influence of the 



bound to it by the force of gravity. They, however, rise at this 

 point till the centrifugal force causing them to recede, becomes 

 balanced by the earth's gravity pulling them back. The side 

 turned towards the sun, however, is moving in a path nearly 

 4000 miles nearer to the sun than that of the earth's centre. 

 Hence, in order to prevent the waters at this place from falling 

 towards the sun, they must move with greater velocity than the 

 centre. But instead of this, they move with less velocity, and 

 the consequence is, the centrifugal force is not sufficient to 

 balance the centripetal force of the sun's attraction. The waters, 

 therefore, fall away from the earth toward the sun, and thus a 

 solar tide is also produced on the side nearest the sun. 



Let us suppose the moon now r to accompany the earth in its 

 orbit. Everything in this case still happens as we have described, 

 with this difference, however, that it is not the earth's centre of 

 gravity, but the common centre of gravity of the earth and moon 

 which now moves in the true orbit. In order to convince our- 

 selves that the heat produced by the solar wave is not altogether 

 at the expense of the earth's diurnal rotation, but must be partly 

 at least at the expense of the earth's rotation on the common 

 centre of gravity of the earth and moon, we have merely to re- 

 flect on what would take place were the earth to be deprived of 

 its diurnal motion of rotation. We should then have a tidal 

 wave produced by the sun the same as before, but it would 

 move round the earth only once in a month. But notwith- 

 standing the slow motion of the wave, the waters would still 

 have to rise and fall the same as formerly, in order to maintain 

 the tidal wave continuously in its true position in relation to the 

 sun, as the earth rotates on its centre 0'. Now the force, which 

 in this case would be converted into heat by the friction of the 

 waters as they rise and fall, could not be derived from the earth's 

 diurnal rotation, for in the present case no such motion exists. 

 Hence we must infer that it is produced solely at the expense of 

 the earth's motion round the common centre of gravity 0' of the 

 earth and moon. 



Suppose that the earth should now begin to rotate on its own 

 axis, it is perfectly evident that the rotation will exercise no influ- 

 ence in the way of preventing the existence of those effects which 

 we have shown take place independently of the rotation. Other 

 effects will, no doubt, follow from the earth's diurnal motion. 

 But those previously occurring will still take place the same as 

 formerly. But if heat is produced at the expense of the earth's 

 motion round the centre of gravity of the earth and moon, 

 then there must be a continual consumption of the vis viva of 

 motion round this centre; for the heat produced can never be 

 all restored to its source. And if the vis viva of the earth's mo- 



