REFLECTING TELESCOPE. 47 



of the mirror towards the extremities, it is made gradually to Arguments and 

 change its spherical form, the part of its area, so abraded and pelting the 

 diminished, cannot subside into a state of actual contact with process of po- 

 the polisher, unless the other parts of it are elevated and disen- lishing mirrors, 

 gaged from the polisher, at the same time ; or unless it may be 

 imagined, that the particles, worn off the mirror by friction, 

 are applied and adhere to the corresponding parts of the 

 polisher, so as to rai^e and augment its surface, just as much as 

 that of the mirror becomes depressed and reduced. If this 

 effect could be supposed to take place, it would follow, that, 

 in every variety in the direction of motion in the mirror, the 

 friction must tend to wear down the edges, rather than the 

 middle of the mirror ; because the motive force is always ap- 

 plied to a part of the handle to which the metal is fastened, 

 raised more or less above the surfaces in contact. The effect 

 of which must be, to communicate to the foremost or advanc- 

 ing half of the mirror's surface, a pressure downward, on the 

 face of the polisher, equal to the force expended in moving 

 the mirror forward ; and thus to abrade and reduce the several 

 parts of the mirror*s surface, proportionally to their respective 

 distances from the center ; by which its curvature will be 

 made to approach to that of a parabola, by its wearing down 

 most towards the edges : and this, weather the motion be con- 

 ducted in lines diametrically across the polisher, or with round 

 strokes ; so as that its center should describe, every time, a 

 little circle, about the center of the polisher. This is, however, 

 entirely on the supposition, that the edges of the polisher 

 become raised, by the adhesion of the dust worn from those 

 of the mirror : for, if this were not the case, but that the po- 

 lisher were to retain its spherical form, while that of the 

 mirror was altered, the contact could not be general between 

 two surfaces of dissimilar shape. If these adhered together in 

 one part, they must be dissevered in another : and the force, 

 . necessary to seperate them in this latter part, which can never 

 be greater than that required to move the mirror forward, 

 must yet be more than equal to the force of cohesion, in the 

 part of the mirror, which, in each stroke, is to be disengaged 

 f/om the polisher. This pressure is found, in the case of 

 bodies in contact, to be incomparably greater than the weight 

 of the atmosphere, which is equal to about seventeen oreigh- 

 teen pounds on every square inch of the surface of the mirro; : 



