PHYSICS OF THE EIGHTEENTH CENTURY. 309 



method consists in rolling the rod between the fingers ; but in many 

 of the Polynesian islands the rod is longer and somewhat flexible, so 

 that it bends under the pressure into a- bow-like form, and the hand 

 applied to its centre imparts to it a circular motion, exactly as a car- 

 penter turns an augur. The method represented in our illustration is 

 that used by the North American Indians : they pass two or three turns 

 of a cord round the rod, and draw the cord alternately in one direction 

 and the other. The reader will hardly fail to remark the identity 

 between the modes of producing heat in Count Rumford's experiment, 

 and in the very ancient and primitive operation just described. 



Rumford estimated the quantity of heat developed in his experiment, 

 taking into account the specific heats of the brass cylinder and of the 

 steel borer, but neglecting that taken up by the wood and lost by 

 radiation. He states the result to be a quantity of heat sufficient to 

 raise 26^ Ibs. of water from the freezing-point to the boiling-point, and 

 this heat, he says, " is produced equably or in a continual stream (if 

 I may use the expression) by the friction of the blunt steel borer 

 against the bottom of the hollow metallic cylinder, and is greater in. 

 quantity than that produced equably in the combustion of nine wax 

 candles each three-quarters of an inch in diameter." The next pas- 

 sages are remarkable as showing that Rumford understood that the 

 source of this heat was the mechanical energy exercised by the horses 

 employed in turning his machine, and that he seems even to have had 

 some idea that the horses' food might in its turn be the source of their 

 muscular power. His words are : " As the machinery used in this 

 experiment could easily be carried round by the force of one horse, 

 though to render the work lighter two horses actually were employed 

 in doing it, these computations show further how large a quantity of 

 heat might be produced, by proper mechanical contrivance, merely by 

 tjie strength of a horse, without fire, light, combustion, or chemical 

 decomposition ; and, in case of necessity, the heat thus produced 

 might be used in cooking victuals. But no circumstances can be 

 imagined in which this method of procuring heat would be advanta- 

 geous ; for more heat might be obtained by using as fuel the fodder 

 necessary for the support of a horse." He then goes on to show that 

 the heat evolved in these experiments could not have been due to a 

 diminished capacity for heat of the fragments which the borer detached 

 from the mass of the brass ; for the explanation which was then com- 

 monly accepted was that the metal thus forcibly detached in small pieces 

 had its capacity for heat diminished, and then the "caloric" oozed 

 out, as it were, in the form of sensible heat. No one before Rumford 

 thought of testing this plausible explanation by experiment : he col- 

 lected the fragments of brass, and found that their capacity for heat 

 was unchanged. He proved also that the heat produced in his ex- 

 periment could in no way have been derived from the air, or the water 

 from other parts of the apparatus. Thus he arrives at the following 



