l68 . PHILOSOPHICAL TRANSACTIONS. [aNN0 1777. 



a rag, and holding it in a pair of tongs over a still fire where there was no rising 

 dust, till it was ready to strain through the linen, I caused it to drop on the 

 several parts of the convex tool, till I supposed it would cover the whole surface 

 about double the thickness of a shilling; then spreading the pitch as equally as 

 I could, I suffered the polisher (by which name I shall for the future call this 

 tool) to grow quite cold. I then warmed the bruiser so hot as almost to burn 

 my fingers, and having fixed it to the bench with its face upwards, I suddenly 

 placed the polisher on it, and quickly slid it oft"; by this means rendering the 

 surface of the pitch more equal. The pitch is then to be wiped off" from the 

 bruiser with a little tow; and by touching the surface with a tallow candle, and 

 wiping it a 2d time, it will be then perfectly clean and fit for a 2d process of the 

 same sort, which must again be performed as quickly as possible; and this is 

 ordinarily sufficient to give a general figure to the surface of the pitch. 

 The bruiser and jjolisher are then suffered to get perfectly cold, when the pitch, 

 considering what has been taken off^, will be about the thickness of a shilling. 



It is however here necessary to observe, that the pitch should be neither very 

 hard and resinous, nor too soft; if the former, it will be so untractable as not 

 to work kindly; and if too soft, it will in working alter its figure faster than the 

 metal, and too readily fit itself to the irregularity of its figure, if it have any. 

 When both tools were perfectly cold, I gave the polisher a gentle warmth, and 

 then fixed the bruiser to the block with its face upwards; and (having with a 

 large camel's-hair brush spread over the face of the polisher a little water and 

 soap, to prevent sticking) with short, straight, and round strokes I worked it 

 on the bruiser, every now and then adding a little more water and soap, till the 

 pitch on the polisher had a fine surface, and the true form of the bruiser; and 

 this I continued to do till they both got perfectly cold together: in this maimer 

 the polisher was perfectly formed in about a quarter of an hour. But here a 

 difficulty arose: when I began to polish the metal, I found that the edge of the 

 hole in the metal collected the pitch towards the middle of the polisher; and 

 though in this method of working I could give an exquisite polish, as the putty 

 lodged itself in the pitch exceedingly well, yet the figure of the metal was 

 injured in the middle, nor did indeed the work go on with that equability which 

 is the inseparable attendant on a good figure. In order to obviate this difficulty, 

 I cast some metals with a continued face, the holes not going quite through, 

 within perhaps the thickness of a six-pence. I finished 2 or 3 metals of this 

 sort, and the work promised and went on very well; but when I aime to open 

 the holes, which I did with the utmost caution, I found the metals short of 

 perfection; which I attributed to an alteration of the figure from the removal of 

 even that small portion of metal after the speculum had been finished. This I 

 suppose was in some measure the reason why I spoiled a very distinct and perfect 



