134 



As the pitch is (in our present enquir)'^) to be considered 

 as an homogeneous substance, we must suppose, that its 

 resisting force, as Avelh as that of the pressure of the mirror 

 on it, are uniformly diifused over the surface of the po- 

 lisher: and, from hence, it may not, perhaps, be easy to 

 conceive, how the surface of the mirror could sustain from 

 it any inequality of Tesistance and friction. In fact, these 

 would be equal and uniform, in every part, if the pitch 

 were a substance, either of perfect hardness, or perfect 

 fluidity: but it will hereafter appear, that its consistence 

 must notibe so hard, as to rendef it incapable of any change 

 of form; but, on the contrary, so soft, as to yield, in a 

 small degree, to the pressure of the mirror: at the same 

 time, opposing a resistance, sufficient to Avear down and 

 polish it: and the enquiry is, how that resistance is mo- 

 dified. > ino'/f 



, Bodies 



in order to establish the true cause, not only to deduce its existence and effects 

 solely fronri reasoning oh physical "principles, but also to obviate other diflerenl 

 conjectures that might be formed, by stating fully those circumstances that take 

 place in this operation; and which, indeed, are necessary to he clearly imder- 

 stood in judicious practice. Both these ends cannot be answered, in a disquisition 

 new and intricate, without a minute explanation: and this, I hope, will be re- 

 ceived as my apology, for the prolixity of this account, which I would gladly 

 have curtailed, if I knew how to do so, without making it less intelligible or 

 useful to the practical optician. This class of readers will forgive any diffusive- 

 ness on a mechanical subject,' if th'6 perusal may terKi to spare them the greater 

 labour of fruitless experiments; or afford any hint towards conducting them 

 more judiciously : and as for their use this paper was designed, I have adverted 

 to such various matters as I thought most worthy their attentioi>; and which yet 

 have not been so fully and familiarly explained by others, a^ they ought to he, 

 for the instruction of an artist. 



