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temper quite hnrd and unyielding, no part of the surface 

 of the mirror can be made to sufter a higher degree of fric- 

 tion than the other parts of it, unless these latter parts be 

 elevated and detached from the face of the polisher, and 

 disengaged from contact with it; because, in this case, both 

 mirror and polisher are supposed to preserve their general 

 shape regular and unaltered: and, therefore, the contact, 

 and, consequently, the friction, must be either complete 

 and equal, on the Avhole surface, or none at all. For, if 

 we suppose, that, by the wearing down of the mirror to- 

 wards the extremities, it is made gradually to change its 

 spherical form, the part of its area, so abraded and dimi- 

 nished, cannot subside into a state of actual contact Avith 

 the polisher, unless the other parts of it ai'e elevated and 

 disengaged from the polisher, at the same time; or unless it 

 may be imagined, that the pai'ticles, worn off the mirror by 

 friction, are applied and adhere to the corresponding parts 

 of the polisher, so as to raise and augTnent 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 applied 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 advancing half of the mirror's surface, 

 a pressure downward, on the face of the polisher, equal 



to 



