145 



to the force expended in moving the mirror forward; and 

 thus to abrade and reduce the several parts of the mirror's^ 

 surface, praportionallj to their respective distances from the 

 center; by Avhich it* curvature will be made to approach 

 to that of a parabola, by its wearing down most towards 

 the edges: and this, whether the motion be conducted 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 polishei*. This is, however, 

 entirely on the supposition,! that the edges of tire polisheT 

 beconae raised, by the adhesion of the dust Avcffn; from those 

 of tb^ mirror: for, if this were not the. case, but that the 

 polisher were to retain its spherical foi-m, while that of the 

 mirror was altered, the contact could not be general be- 

 tween two surfaces of dissimilar shape. If these adhered 

 together in one part, they must: be dissevered in another; 

 and the force, necessary to separate them in this latter part, 

 Avhich can never be greater than that required to move the 

 mirror forwai-d, must yet be more than equal to. the force 

 of cohesion, in the past of the mirror, which, in each stroke, 

 is to be disengaged from 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 eqtta! 

 to about seventeen or eighteen pounds on every square inch 

 of the surface of the mirror: and, when this latter is brought 

 so near that of the polisher, as to suffer friction from the 

 powder bedded in it, their mutual attraction will anioutit 

 to a much greater force than is requisite to move forward 



the 



