133 



on the subject, by Messrs. Hadley, Mudge, Edwards, Sec.;) 

 and I was confirmed in my suspicion, from the following 

 reasons, after I had found them approved by many re- 

 peated and diversified experiments. 



Pitch is a soft unelastic substance, which, as such, will 

 suft'er a permanent change of form, Avhen it is made to 

 sustain a degree of pressure sufficient to communicate an 

 intestine motion to its particles: and this property directs 

 us to consider, what may be the eft'ect of the pressure of 

 the mirror on it, when spread on the polisher, as to the 

 figure it may then gradually acquire, during the operation 

 of polishing, and the resistance and friction it will oppose 

 to the mirror: for, by reason of the tenacity of its sub- 

 stance, it will resist a certain degree of pressure, without 

 change of its form, but will yield to a greater pressure. 

 But it is by its resistance the mirror is worn down and 

 polished; if, therefore, that resistance be not uniform and 

 equal, on the Avhole surface of the polisher, neither will 

 the abrasion of the mirror be equal in every part; the 

 consequence of which must be, that both will degenerate 

 from an uniform curvature, i. e. from a spherical figure; 

 the mirror from vmequal friction, and the polisher from its 

 mobility, by which it will adapt itself to the successive 

 alterations produced in the figure of the mirror; their mu- 

 tual action and reaction inducing a change in both.* 



VOL. X. s As 



* This change, however, being so little, as to be imperceptible by the senses, 

 and, in the imagination, referable to various other causes, it becomes necessary. 



