l06 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777, 



most trifling error here will be sufficient to spoil the figure of his metal, and 

 render all his preceding caution useless. I have however discovered a method 

 which I shall explain, not only of giving the metal a parabolic figure, but also 

 of recovering it when it happens to be injured; both to be effected in the act of 

 polishing, and the former as certainly as the spherical figure is given on the 

 hones. Indeed, if we consider rightly, polishing will be perceived to be but a 

 kind of grinding with a finer order of strokes, and with a power infinitely finer 

 than was before used in what is commonly called the grinding. But before 

 describing this method, which was the result of many years experience, I will 

 take the liberty of making some few strictures on that of Messrs. Hadley and 

 Molyneux, which is followed by the generality of workmen. 



First, then, the tool itself used by them for polishing the metal, is formed 

 with infinite difficulty. The first described polisher is directed to be made by 

 covering the tool with sarsenet, which is to be saturated with a solution of pitch 

 in spirit of wine, by successive applications of it with a brush, till it is covered, 

 and by the evaporation of the spirit of wine filled with this extract of pitch; the 

 surface is then to be worked down and finished with the bruiser. This is all very 

 easy in imagination; but whoever has used this method (which I have myself 

 unsuccessfully several times) must have found it attended with infinite labour, 

 and at last the business done in a very unsatisfactory manner; for the pitch by 

 this process will be deprived of an essential part of its composition. The spirit 

 of wine dissolves none but the resinous parts of its substance, which is hard and 

 untractable; and if you use soap or spirit of wine to soften or dissolve it, it will 

 equally afitct the whole surface, the lower as well as higher parts of it. And 

 suppose that with infinite labour with the bruiser, it is at last reduced to a fine 

 uniform surface, it is nevertheless too hard ever to give a good polish with that 

 lustre which is always seen in Mr. Short's, and indeed all other good metals. 

 Nor will it give a good spherical figure; for a perfect sphere is formed, as before 

 observed, by that intimate acconunodation arising from the wear and yielding of 

 both tool and metal; whereas in this method, there is such a stubbornness in the 

 polisher, that the figure of the metal, good or bad, must depend on the truth 

 of the former, which is very seldom perfect. 



If the polisher be made in the '2d manner proposed, by straining the pitch 

 through an outer covering, which is afterwards to be stripped oft', the superficies 

 of pitch and sarsenet is so very thin, that the putty, working into them, forms 

 a surftice hard and untractable, so that it is impossible to give the speculum a fine 

 polish. Accordingly all those metals which are wrought that way have an order 

 of scratches instead of polish, discovering itself bv a greyish visible surface. 

 Besides, supposing this tool perfectly finished, and answering its purpose ever so 

 well, it is possible it can produce in the speculum any other than a spherical 



