JOURNAL 



O F 



NATURAL PHILOSOPHY, CHEMISTRY, 



AND 



THE ARTS. 



MARCH 1800. 



ARTICLE I. 



B'trtBions for making the bejl Ccmpqfition for the Metals of RtJleBing TelefcoptSt and the 

 Method of Cajlingy Grinding, Polijhingt and giving the great Speculum the true parabolic 

 Figure. By the Rev. John Edwards, B. A. 



( Concluded from page 497.^ 



Of polijljing the Metal, and giving it the true parabolic Figure. 



T. 



HE rough grinder, of an elliptical form, is now to be covered with common pitch. I 

 generally make my own pitch by boiling tar in a ladle or crucible over a very flow fire, till 

 it becomes of the confiftence I require; for a greater nicety is required in the degree of the 

 hardnefs of the pitch. The harder the pitch is the better figure it will give to the metal, as 

 it does not alter its figure in working as foft pitch does ; befides the metal will acquire a 

 luftre upon a polifher moderately hard, fo as to (hew objetSs refle£led upon it as vivid and 

 as near their natural colour as poflible ; but if the pitch is too foft, fome of its fineft particles 

 will always adhere to the face of the metal, and form a very fine and thin cuticle or cover- 

 ing upon its furface. This circumftance is rendered very evident by viewing any white 

 objedl in the metal (a flieet of white paper for example) when that fine cuticle or thin fur- 

 face of the pitch upon the fpeciilum will caufe it to ihew the objeft of a dingy brown colour. 



Vol. Ill,— March 1800. 3Y and 



