135 



Bodies of perfect hardness, such as glass, flints, &c. will 

 not admit a total intimate change of their form, in all their 

 dimensions, Avithout a dissolution and permanent separation 

 of all the particles composing their masses, (except when 

 they are brought to a state of fusion by heat). But soft, 

 viscid, semifluid bodies, such as lead, pitch, &c. will sufter 

 such change, preserving the cohesion of their particles, yet, 

 at the same time, undergoing a general intestine motion of 

 all the particles among themselves: so that the coat of 

 pitch, pressed, on each side, between the parallel surfaces 

 of the mirror and polisher, will, by their force, be equally 

 extended laterally in every direction; by which an equal 

 quantity of motion will be. communicated to all its parti- 

 cles: since no . particles, except those at the extremities^ 

 can move, -without protruding others, and these, the resty 

 successively,' as af the mass were.' a fluid body. 



But, though all parts of the surface of the polisher receive 

 an equal pressure and motion, all do not exert an equal 

 degree of resistance to that pressure: for those parts, that 

 cannot move without displacing and overcoming the resist- 

 ing tenacity of a greater quantity of the surrounding mass of 

 pitch, than other parts do, must oppose the greater resistance 

 to the mirror, as having that of the other parts superadded 

 to their own. For ascertaining this, the force impressed, and 

 the quantity of pitch, confining any annular tract of the 

 polisher, should be computed. In the present case, where 

 the coat of pitch is a thin equal stratum, of circular form, 

 we need regard only its superficial dimension, and consider 

 , . s 2 all 



