51 



the earth becomes the planet in his stead, all that was true be- 

 fore, in respect of his rotation and revolution, becoraes/aZ^e now ; 

 and it is from overlooking the consequences of this change of cir- 

 cumstances, that the error I am combating seems to have origi- 

 nated ; and to this confounding of ideas, this mistaking of things 

 different for precisely the same, the long continued prevalence 

 of so very palpable a set of errors is to be ascribed 



Having, therefore, now finished my own case, and brought 

 forward such arguments, proofs, and illustrations, as seem suffi- 

 cient in my judgment to establish the truths or what I believe to 

 be the truth, I shall now proceed to examine such of the princi- 

 pal arguments and illustrations usually brought forward in sup- 

 port of the received opinion, as I have not yet had occasion to 

 notice. I might, indeed, soon make up a large volume with the 

 materials that superabound upon this subject, but I must be con- 

 tent at present with such as seem to be of sufficient importance 

 to deserve notice. With respect to the principles of the whole 

 question, the invariableness of relation of a planet's or satellite's 

 axis and radius-vector to the primary's centre, and the ascription 

 of every change of relations between the moving body's circum- 

 ference and its radius-vector, exclusively to the rotatory move- 

 ment of the revolving body itself, I have never had the smallest 

 doubt or wavering since I read and understood Galileo's illus- 

 tration of the subject, which I shall now present to my readers. 



In his System of the World, Dialogue iii. he says, " Naturally, 

 without any moving cause^ a body freely suspended and carried 

 round the circumference of a circle, immediately of itself acquires 

 a rotation^ that carries it round its own centre in a direction 

 contrary to the motion which carries it round the circle, and with 

 such velocity that both of these motions of revolution and rota- 

 tion are completed in precisely the same time. This may be ad- 

 mirably illustrated by the following experiment Take in your 

 hand a basin of water ; and turn yourself round on your feet, 

 and you will see the water begin to turn itself round in a direc- 

 tion contrary to that of the basin, and finish its rotation at the 

 same time with your and the basin's revolution. But, if you 

 consider the matter attentively, you will perceive that the eflPect 

 produced is nothing real, but only a mere appearance ; that, what 

 seems to be a rotation on its own axis, is really no motion at all; 

 and that the water remains immovable in respect of every thing 

 stationary beyond the limits of its orbit ; presenting the same side 

 or hemicycle continually to the same side of the room, or field, 

 where the experiment is made, or to the same part of the sky. If, 

 on the other hand, it be compared with the basin and yourself, 

 who are both moving, it will be observed to change its direction, 



