1 ^ : The m:Jfm DoBrine »f Heat applied to the Art of Gnni'vig. 



verifed hone is occafionally applied by n>eans of tallow. The peculiar advantage of this 

 kind of pottery was flated to be, that it never heats, however rapid the motion. 



Tiiis objecl fecmed to deferve an experimental inveftigation. The three bodies fub- 

 jedcd to experiment were pottery, pulverifed fdex, and tallow. The effcfl, namely thnt 

 it could fupport violent and rapid fridion witliout increafe of temperature, appeared at 

 firft fcarcely credible. The mind was rather difpofcd to reject the evidence, than inveftigate 

 the caufe. No indication was given Tefpe£ling the nature of the pottery. It did not feem 

 probable that any peculiarity in the filiceous fand Ihould produce this defirable effeifl ; but 

 an eafy proccfs of reafoning might have pointed out the effeft of the tallow, which is 

 indeed curious. I cannot however aflume the merit of inveftigating the fubjedl h prior! ; 

 for my apparatus was ready for experiment, and the fa£l fpoke for itfelf before I had 

 fyflematifed the notions which occurred to me. 



The pottery grindflone was not cafily attainable. I therefore procured a Newcafllc 

 grindftone of a fine grit and ten inches in diameter ; and alfo a block of mahogany to be 

 ufed with emery on its face. Both the (lone and the wooden block were mounted on an 

 axis to be occafionally applied between the centres of a ftrong lathe. In this fituntion both 

 were turned truly cylindrical, and of the fame diameter. The face of the wood was grooved 

 obliquely in oppofite dire£lions, to afford a lodgement for the emery. The face of the (lone 

 was left fmooth, and there was a trough of a proper fize applied beneath the (tone to hold 

 water. The grindflone was then ufed with water, and the wooden cylinder was faced 

 with emery ?:id oil. The inftrumcnt ground was a file, out of which it was propofed to 

 grind all the teeth. The rotation was produced by the mechanifm of the lathe ; the velo- 

 city being fuch as to turn the grinding apparatus about five revolutions in a fecond. The 

 ftone operated buc flowly, and the water from the trough was foon exhaufled, with incon- 

 venience to the workman, who could fcarcely be defended from it but by flackening the 

 velocity. The emery cylinder cut rather fafler. But notwithflanding the fri£lion was 

 made to operate fuccefTively and by quick changes on the whole furface of the file, it foon 

 became too much heated to be held with any convenience ; and when a cloth was ufed to 

 defend the hand, the work not only became awkward, but the heat increafed to fuch a 

 degree that the oil began to be decompofed, and emitted an empyreumatic fmell. The flonc 

 was then fuflFered to dry, and the file tried upon its face. It almoft immediately became blue, 

 and foon afterwards red-hot. Both the cylinders were then covered with tallow, by ap- 

 plying the end of a candle to each while revolving, and emery was fprinkled upon the 

 cylinder of wood. The fame tool was then applied to the grindllone in rapid motion. 

 At the firft inflant the fri£lion was fcarcely perceptible ; but very fpeedily afterwards the 

 zone of tallow prefTed by the tool became fufed, and the (lone cut very fall. The tool 

 was fcarcely at all heated for a long time ; and when i*- began to feel warm, its temperature 

 was immediatly lowered by removing it to a new zone of the cylinder. The fame eifeft 

 took place when the experiment was repeated with the wooden qylinder. 



It is not difficult to explain this by the modern doclrine of heat. When oil was ufed 

 upon the wooden cylinder, the heat developed by the fri£lion was employed in raifing the 

 temperature of the tool and of the fluid oil. But when tallow was fubllitutcd inftead of the 

 oil, the grcatefl part of the heat was employed in fufing this confident body. From the 

 increafed capacity of the tallow, when melted, this heat was abforbcd, and became latent, 



inftead 



