30 OUTLINES OF NATURAL PHILOSOPHY. 



This has been explained under the heads of the first 

 law of motion, and of the distinction between ab- 

 solute and relative motion. 



41. The reality of the diurnal revolution of 

 the heavens, is liable to a great objection, as 

 it supposes that a circular motion, in the same 

 direction, is common to an immense number of 

 bodies, far distant, and entirely detached from 

 one another ; and that this motion is so regula- 

 ted, that their revolutions are all performed in 

 the same time, and in planes parallel to one 

 another. 



The revolution of a detached body about a centre, or 

 about an axis, cannot take place without a force 

 constantly acting, to draw it out of the straight 

 line in which it has at every instant a tendency to 

 continue its motion. The revolution of a solid 

 body, like the earth, on its axis, may arise from 

 one original impulse ; its continuance requires no 

 new action, but is a consequence of the inertia of 

 matter. 



It was to obviate the difficulty arising from the de- 

 tached and distant situation of the bodies, to which 

 motions so perfectly coincident were ascribed, 

 that the hypothesis of crystalline orbs was in- 

 vented. To those who do not admit the exis- 

 tence of these orbs, the diurnal revolution of the 

 heavens can have no probability. 



42. The 



