234 COSMICAL ARRANGEMENTS. 



menon, while all the other motions of the system 

 are subject to inequalities? How is it that in 

 the celestial machine no retardation takes place 

 by the lapse of time, as would be the case in any 

 machine which it would be possible for human 

 powers to construct ? The answer is, that in the 

 earth's revolution on her axis no cause operates 

 to retard the speed, like the imperfection of 

 materials, the friction of supports, the resistance 

 of the ambient medium ;* impediments which 

 cannot, in any human mechanism, however per- 

 fect, be completely annihilated. But here we 

 are led to ask again, why should the speed con- 

 tinue the same when not affected by an extra- 

 neous cause ? why should it not languish and 

 decay of itself by the mere lapse of time ? That 

 it might do so, involves no contradiction, for it 

 was the common, though erroneous, belief of all 

 mechanical speculators, to the time of Galileo. 

 We can conceive velocity to diminish in pro- 

 ceeding from a certain point of time, as easily as 

 we can conceive force to diminish in proceed- 

 ing from a certain point of space, which in 

 attractive forces really occurs. But, it is some- 

 times said, the motion (that is the velocity) must 



* It has already been stated that the resisting medium spoken 

 of in Chapter VIII. of this Book has not yet produced any effect 

 which can be detected in the motion of the earth. Probably the 

 effect of this medium upon the rotation of the earth would be ex- 

 tremely small compared with its effect on the earth's motion in her 

 orbit ; and yet this latter effect bears no discoverable proportion to 

 the effect of the smallest perturbing forces of the other planets. 



