^,g REFLECTING TELESCOPE. 



ATgumeiitsand and, when (his latter is brought so near that of the polisher^ 

 inferences res- ^^ ^^^ suffer friction from the powder bedded in it, their mutual 

 pecting the . .,, ^ , ^ , . . . 



vprocess of po- attraction wul amount to a much greater lorce than is requisite 

 iishuis mirrors, to move forward the mirror : no part of which can, therefore, 

 be disengaged from the polisher, nor, consequently, be une- 

 qually 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, whether 

 it be supposed, that the force of cohesion is confined to the 

 very surfaces in contact, or extends to a little distance from 

 them, diminishing in the duplicate, or any other ratio of that 

 distance ; and that the bodies are not wholly removed out of 

 the sphere of attractiou when there is a small interval between 

 them. For, as this force is greatest at the very surface j 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 surfaces in contact 

 with each other. But this pressure acts as much upon the 

 coat or plate of water, which must be interposed between the 

 surfaces of the mirror and polisher, as upon these surfaces 

 themselves ; and, because the pressure upon any part of a 

 confined fluid, is propagated to the whole of it, in every di- 

 rection ; so, the weight of the atmosphere, resting on the 

 edges of this fluid plaie, tends as much, by the interposition of 

 the same, to buoy up, and force assunder, the surfaces resting 

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

 its action on themselves ; and exerts itself equally to prevent 

 their approach on one side, as their recession on the other. I 

 conceive the agency of these forces to be this ; that the plate of 

 water is so strongly attracted by she surfaces nearly in con- 

 tact, as to be kept from running off, and has its outer edge 

 exposed to the weight of the air ; whose pressure is thus cora- 

 iminicated to ail the particles of the water, and, by its me- 

 diation, to the contiguous surfaces of the mirror and polisher. 

 And, though all these are really compressed together, by the 

 surrounding atmosphere, yet I conceive that this does not 

 hinder their gradual separation from being effected : because,'^ 

 as fast as that separation takes place on any side, the air and 

 water rush in betvs'eenthe s*rfaees, to fill up the vacuity, as 



