382 Simpering of Sled b) Oil.—R:f,-J Shot. 



piece upon fund to be gradually heated, or upon melted lead. The faw-makers, and fonie 

 makers of fprings, heat the article, rub it with gvcafe, and then heat it ftill farther till the 

 fumes take fire ; this is called blazing, and affords a temper nearly the fame as when the 

 ftecl by heat has acquired a deep blue colour, ^^'hcn the temper is given from the colour, 

 the firfl. tinge which appears is a f.iint draw colour, which is fuitableto pen-knives and hard 

 cutting-tools. The next colour, whicliis purple, is rather too foft for a knife, and too brittle 

 for a fpring. Aher this follows tlie blue, of whicli there are feveral fliadcs. The docpeft 

 is very foft, and is fuccccded by a whitifli yellow, which indicates too great a degree of foft- 

 ncfs for any cutting-tool. 



Mr. Hartley, in the year.1789, took out a patent for a method of tempering (le^l. His 

 fpccification, which is fo general as perhaps to include no method at all in the way of 

 monopoly, indicates that the he^t is to be meafured by a pyrometer or thermometer ap- 

 plied near the article. The ailual prac£Vice of this method appears to confill in ufing oil and 

 a mercurial thermometer. In this way many dozens of razors or tools may be tempered at 

 once with the utmoft facility. The different degrees of heat for various kinds of fteel, and 

 tlieir feveral ufes, may fpeedily be determined by experiment. For want of a tliermometer 

 graduated to the higher degrees, I have not yet made any experiments. The only fa£l I 

 have at prefent to communicate is, that Mr. Stodart, who ufes this method, ftates that the 

 requifite temper on fteel for a pen-knife is 450 degrees of Fahrenheit's fcale *. 1 find, or> 

 trial, that a good pen-knife is as hard as any tool which can admit of tempering. Hard 

 gravers, for turners' ufe, muft not be foftened at all. 



3. Rijled Shot. 



IN the latter end of the year 1789, I was, by various confidcrations, induced to think, 

 that the effetl which is produced by rifling mufquetry might be produced in artillery by 

 giving a fuitable figure to the (hot. It is almoft ne:^dlefs to explain this effect. When a 

 bullet is driven along the bore of a piece, it muft be adled upon by the internal furface fo as 

 to caufe a rotation, the axis of which motion will lie acrofs the line of direction. In con- 

 fequence of this, the rc-aclion of the air will be ftronger on one fide of the bullet than 

 on the other, and it will deviate from the intended courfe according to no certain rule. 

 The method of rifling eonfifts in cutting one or more fpiral grooves in the hollow furface 

 of the mufquct, into which the ball is either forcibly rammed down, or clfe conveyed to its 

 place by an aperture at the breach, or near the chamber. The lead is thus made to fit the 

 internal fcrew, and ufually takes about half a turn during its courfe through the barrel. 

 The axis of this rotation being parallel to the line of 'direclion, it mufl follow that the 

 refinance of the air will be equal on all fides of the bullet, and it will fly with 

 more certainty to the obje£l of aim. It feemed to me, that if a cylindrical fliot, with 

 hcmifpherical ends, were thrown out of a common barrel, it might be poffible, by means 

 of certain fpirals cut on the end furface, to caufe the blafl of the powder and the refiftance 

 of the air to concur in producing the fame rotation. 



For this purpofe I took a wooden pattern, an.d cut the fphcrical furface into twelve fpiral 

 planes, by dividing the equator into the like number of equal parts, and drawing fpirals from 



• Philofophical Tranfaflioiu '7l;>r. 326. 



the 



