ON THE SOURCES AND EFFECTS OF HEAT. 485 



observing the velocity with which the thermometer rises, Mr. Dalton has 

 estimated that air, compressed to half its dimensions, has its temperature 

 elevated about 50 degrees of Fahrenheit ; and some of his experiments 

 indicate, when accurately examined, a still greater change.* For the 

 present we may define the sense of the term degree, in Fahrenheit's scale, 

 as corresponding to an expansion of a portion of mercury amounting to 

 one ten thousandth part of its bulk ; and a degree of Reaumur originally 

 corresponded to an expansion of a weak spirit of wine, amounting to one 

 thousandth part of its bulk. It may be inferred from the velocity of 

 sound, supposing that the excess of its velocity above the common calcula- 

 tion is wholly derived from the heat and cold produced by condensation 

 and expansion, that a condensation amounting to -r^- of the bulk of any 

 portion of air will raise its temperature one degree of Fahrenheit. When 

 air is very rapidly compressed in the condenser of an air gun, it is some- 

 times so much heated as actually to set on fire a small portion of tow, 

 placed near the end of the barrel, t 



The production of heat by friction is too well known to require an 

 experimental proof; but Count Rumford has taken particular pains to 

 ascertain every circumstance which can be supposed to be concerned in 

 the operation of this cause ; and the results of his experiments are so 

 striking, that they deserve to be briefly related. He took a cannon, not 

 yet bored, having a projection of two feet beyond its muzzle, a part which 

 is usually cast with the piece, in order to insure the solidity of the metal 

 throughout, by the pressure which its weight occasions. This piece was 

 reduced to the form of a cylinder, joined to the cannon by a smaller neck, 

 and a large hole was bored in it : the whole cannon was then made to 

 revolve on its axis by means of the force of horses, while a blunt steel 

 borer was pressed against the bottom of the hollow cylinder, by a force 

 equal to about 10,000 pounds avoirdupois ; the surface of contact of the 

 borer with the bottom of the cylinder being about 2 square inches. This 

 apparatus was wrapped up in flannel, when its temperature was about 60. 

 In half an hour, when the cylinder had made 960 turns, the horses being 

 stopped, a mercurial thermometer was introduced into a perforation in the 

 bottom of the cylinder, extending from the side to the axis, and it stood 

 at 130, which Count Rumford considers as expressing very nearly the 

 mean temperature of the cylinder. The dust or scales, abraded by the 

 borer, weighed only 837 grains, or about -^ of the whole weight of the 

 cylinder. In another experiment, the cylinder was surrounded by a tight 

 deal box, fitted w r ith collars of leather, so as to allow it to revolve freely, 

 and the interval between the cylinder and the box was filled with 19 

 pounds of cold water, which was excluded from the bore of the cylinder 

 by oiled leathers fixed on the borer ; and after two hours and a half, the 

 water was made to boil. Hence Count Rumford calculates that the heat 



* Manch. Mem. v. 515. 



f On the production of heat by condensation, and cold by rarefaction, see Dar- 

 win, Ph. Tr. 1788, p. 43; Pictet, Jour, de Phy. xlvii. 186; Baillet, ibid, xlviii. 

 166 ; Ph. Mag. xiv. 363. 



