149 



superadded, when no fluid is interposed: all Mliicli shew 

 that their cohesion, when a fluid does intervene, is not 

 caused by the pressure of the atmosphere. 



Agreeably to this, the sagacious Newton directs, that, 

 toward the end of the operation, no more moisture 

 should be applied to the polisher, than what it will con- 

 tract, from the operator's breathing on it. Indeed, a per- 

 son, who has formed a just conception of his genius and 

 intense application of mind, and considered the hints and 

 precepts he has given in this Avork, can hardly doubt, 

 that he could, and, perhaps, Avould, have furnished a 

 theory of the rules and method of this whole process; 

 had he not imagined it Avould, at that time, be re- 

 garded as a matter of too little importance, to deserve 

 so minute an explanation, which must hp necessarily 

 prolix, and seem unworthy of him, who was occupied in 

 more sublime speculations. ,^ 



From this it follows, that, when the pitch is of un- 

 yielding hardness, it will not, in any mode of polishing, 

 communicate to the mirror the desired shape, if the dust, 

 worn from the mirror, does not alter the shape of the 

 polisher. And, as this seems not likely to happen, so I 

 was not surprised, that my efforts, to effect the desired 

 figuration of the mirror, by using very , hard, and refrac- 

 tory pitch, failed of success, .,.' . '. 



And there is this inconvenience, moreover, in the use 

 of such pitch, viz. that jit makes so great resistance to 

 the sinking and bedding of the polishing powder in it, 



VOL. X. V that 



