Speculums of Rejlening Telejtopts, 495 



knitting needle, to permit the air to efcape as the metal is poured into the mould. I have 

 found by experience thatfeveral fmall holes are infinitely better for that purpofe* than one 

 large hole. When the metal is melted a fecond time, which mud be done with as fmall a 

 degree of heat as poflible, add the proportional quantity of crude arfenic in coarfe powder, 

 and ftir it well with a wooden fpatula ; when the fumes are gone oiF, take the metal off the 

 fire, take away the drofs, and add half an ounce or an ounce of powdered rofin, or equal 

 parts of powdered rofin and nitre, in order to give the metal a good face ; ftir it well 

 with a ftick, and pour it immediately into the flafks. When the git is filled up with the 

 fluid metal, ftrike the flaflcs gently, fo as to fliake or jog the metal in them in a fmall de- 

 gree i this will prevent any flaws in the face from any air-bubbles being lodged there. 

 When the metal has remained in the flalks for a few minutes, fo as to become entirely folid, 

 open the flalks while the metal is red hot (it cannot crack in this ftate, though it is expofed 

 to the air, as all metals are malleable when they are red hot) and take out the fpeculum with 

 a pair of tongs, laying hold of it by the git, but take care to keep the face downwards, to pre- 

 vent it from finking. Force out the fand from the hole in the middle of the mirror with a 

 piece of wood or iron, and place the fpeculum in an iron pot, with a large quantity of hot 

 aflies or fmall coals, fo as to bury the fpeculum in them a fuflicient depth. If the fand is 

 not forced out of the hole in the manner above diredled, the metal, by finking as it cools, 

 will embrace the fand in the middle of the fpeculum fo tight, as to caufe it to crack before it 

 becomes entirely cold. And if the metal is not taken out of the fand, and put in a pot with 

 hot allies or coals to anneal it, I can affert, that the moifture from the fand will always 

 break the metal. Let tlie fpeculum remain in the aflies till the whole is become quite cold. 

 The git may be eafily taken off" by marking it round with a common fine half round file, and 

 giving it then a gentle blow. The metal is then to be rough ground and figured. 



Of the rough grinding and figuring the Speculum. '" 



In rough grinding, figuring, and polifliing the metal, two tools only are necefl"ary befides 

 a common grind ftone. One chief reafon why workmen do not give a good figure to their 

 fpecula is, becaufe by purfuing Dr. Smith's or Mr. Mudge's method, they ufe too many 

 tools, which, in a great meafure, deftiroy each other's effefts. As nature always aQs in the 

 moft fimple manner, fo if we could always imitate her in this refped, we fhould arrive at a 

 much greater degree of excellence in mofl of our mechanical purfuits. Befides the tools 

 generally made ufe of by workmen are confiderably too large in diameter ever^o give a diredl 

 and uniform figure. All the tools I make ufe of are a rough grinder, compofed of lead and 

 tin mixed together, or elfe of pewter ; this rough grinder ferves alfo for a poliflier ; this 

 tool, with a bed of ftones or hones, are all that are neceflary. A bruifer (as dire(3:ed by 

 Dr. Smith and Mr. Mudge) is totally unneceflary, caufes confiderably more work, and, 



* If feveral fmall holes are made for the air to efcape, the back of the metal will be caft much neater than 

 if one hole only is ufed for this purpofe. Befides, when one hole only is ufed to let the air efcape, the metail 

 is very apt to crack in that place, owing to the finking of the metal in cooling. 



. 3 S 2 after 



