l60 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777- 



melt the copper alone, so it was not sufficient to calcine the tin ; the speculum 

 was then perfectly close, and free from this fault ; nor did I ever after, in a 

 single instance, meet with the abovementioned imperfection. 



All that is necessary therefore to be done, to procure a metal which shall be 

 white, and as hard as it can be wrought, and also perfectly compact, is to melt 

 2 lb. of Swedish copper, and when so melted, to add 14^ oz. of grain-tin to it ; 

 then, having taken oft' the scoria, to cast it into an ingot. This metal must be 

 a 2d time melted to cast the speculum ; but as it will fuse in this compound state 

 with a small heat, and therefore will not calcine the tin into putty, it should be 

 poured oft' as soon as it is melted, giving it no more heat than is absolutely 

 necessary. It is to be observed hov\ever that the same metal, by frequent melt- 

 ing, loses something of its hardness and whiteness : when this is the case, it 

 becomes necessary to enrich the metal by the addition of a little tin, perhaps in 

 the proportion of half an ounce to a pound. And indeed when the metal is first 

 made, if instead of adding the 14-1- oz. of tin to the 2 lb. of melted copper, 

 about one ounce of the tin were to be reserved and added to it in the succeeding 

 melting, before it is cast oft' into the moulds, the composition would be the 

 more beautiful, and the grain of it much finer: this I know by experience to be 

 the case. The best method for giving the melted metal a good surface is this : 

 the moment before it is poured off, throw into the crucible a spoonful of char- 

 coal dust ; immediately after which the metal must be stirred with a wooden 

 spatula, and poured into the moulds. 



The metal being cast, there will be no occasion for the complicated apparatus 

 directed by Dr. Smith, for grinding and polishing it. Four tools are all that are 

 necessary, viz. the rough grinder to work oft' the rough f;ice of the metal ; a 

 brass convex grinder, on which the metal is to receive its spherical figure ; a 

 bed of hones which is to perfect that figure, and to give the metal a fine smooth 

 face ; and a concave tool or bruiser, with which both the brass grinder, and the 

 hones are to be formed. A polisher may be considered as an additional tool ; 

 but as the brass grinder is used for this purpose, and its pitchy surface is expe- 

 ditiously and easily formed by the bruiser, the apparatus is therefore not enlarged. 

 Of rough-grinding the speculum. — The tool by which the rough surface oi the 

 metal is rendered smooth and fit for the hones, is best made of lead, stitt'ened 

 with about a 5th or 6th part of tin. This tool should be at least a 3d more in 

 diameter than the metal to be ground ; and for one of any size, not less than 

 an inch thick. It may be cemented on a block of wood, to raise it higher from 

 the bench. This leaden tool being cast, it must be fixed in the lathe, and turned 

 as true as it is possible, by the gage, to the figure of the intended speculum, 

 making a hole or pit in the middle, as a lodgment lor the emery, of about an 

 inch diameter for a metal of 4 inches : when this is done, deep grooves must be 



