146 



tlic mirror; no part of which can, therefore, be disen- 

 gaged from the polisher, nor,- consequent!)', be unequally 

 worn down, so as to produce, in its surface, a form dif- 

 ferent from a spherical one, or from that of the polisher. 



This reasoning and conclusion will equally stand, Avhe- 

 ther it be supposed, that the force of cohesion is con- 

 . fined to the very surfaces in contact, or extends to a 

 little distance from them, diminishing in the duplicate, 

 or au3' other xatio of that distance; and that the bodies 

 are not wholly removed out of the sphere of attraction, 

 when there is a small interval between them. For, as 

 this force is greatest at the very surface; so, the bodies 

 in contact cannot be disjoined at all, to the smallest 

 distance, but .by a force superior to the whole cohesive 

 force- , 



It may, perhaps, be imagined, that the pressure of the 

 atmosphere ought to be taken into consideration, and be 

 added to the force of cohesion, which keeps the sur- 

 faces in contact viiih each other. But this pressure acts 

 as much upon the coat or plate of water, which must 

 be interposed between the surfaces of the minror and 

 polisher, as upon these surfaces themselves: and, because 

 the pressure, upon any part of a confined fluid, is pro- 

 pagated to the Avhole of it, in every direction; so, the 

 weight of the atmosphere, resting on the edges of this 

 fluid plate, tends as much, by the interposition of the 

 jjame, to buoy up, and force asunder, the surfaces rest- 

 ing on it, as it does to compress together these surfaces, 



by 



