150 



that the particles of 'the'' 'powder, however fine it may 

 be, will, on any fresh application of it, or when any 

 grains of it are accidentally dislodged from the pitch, roll 

 about loose on the polisher, and scratch the face of the 

 mirror, so as to destro^f'" thfe' polish before given; thus 

 making any fresh 'application of the powder inadmissible, 

 unless the pitch Avere to be softened by heating it, which 

 would destroy its fotmer figure, and render the opera- 

 tion uncertain and tedious, it Avas to allow the polish- 

 ing powder to fix itself, without rolling loose on the po- 

 lisher, and to suffer all its particles, however different 

 in size, to sink in it, so as to form an even sui-face, that 

 Sir Isaac Newton, in his sagacity, employed a coat of 

 pitch on the polisher, as a soft substance, that Avould yield 

 to the p'owdct, when impressed on it by the mirror, and 

 not afford siicb' rfesistaiice, as to make it fret the face of 

 the metal; and also as a substance endued with another 

 property equally necessary, that of being perfectly un- 

 elastic. For no elastic substaftce will ever communicate 

 an ^xqMsite polish to a metallic speculum, though it Avould 

 td' glbW,' 'Crystal, or jewels; because no metal can be cast, 

 ■perfebtly free from sihall pores: and any elastic substance, 

 if employed to polish it, would insinuate itself, together 

 with the polishing powder, into these pores,' and wear 

 down their edges in such a manner, as to convert every 

 pore into a' long furrow or cavity; which ' Avoukh occasion 

 tlie destruction of the whole surface of the metal, as was 

 truly obscn^ed by Sir Isaac NeAvton. And thus it ap- 

 ' " pears. 



