2 Professor Dewar [Jan. 24, 



and in looking back at his work in the light of present knowledge, 

 it seemed simply astounding that he should have succeeded so com- 

 pletely as he did. The original coil used by Joule for the mechanical 

 determination of heat (kindly lent for the occasion by Prof. Eiicker) 

 was shown, and the course of the experiment explained. The vast 

 difficulties which Joule had to overcome in order to prove that 

 there was a definite, permanent, and persistent relation between 

 the amount of mechanical energy expended and the heat pro- 

 duced were commented on ; the thermal efiects being produced not 

 directly but through the medium of an electric current varying 

 in intensity, and calculations having to be made not only for these 

 fluctuations, but for the effects of radiation, the movement of the air, 

 and other indirect complications. The very small increment of heat 

 to be measured obliged Joule to use thermometers of great delicacy, 

 and these he had to devise and construct himself. One of the 

 thermometers so used was exhibited. 



Working in this way, Joule was able by the end of July, 1843, to 

 state definitely that the amount of heat capable of increasing the 

 temperature of a pound of water by 1° F. was equal to, and might be 

 converted into, a mechanical force capable of raising 838 lbs. to the 

 height of one foot. Soon afterwards he attained almost identical 

 results by a more direct method — the friction of water passing through 

 small tubes — which gave him 770 foot pounds per unit of heat. 



It was impossible, said the lecturer, to thoroughly appreciate 

 Joule's work without glancing at the early history of the subject ; 

 and when one did so it was amazing to find how near men of the 

 stamp of Eumford, Davy, and Young had been to Joule's great dis- 

 covery, and yet missed it. Count Eumford was the first to clearly 

 define the relation between the constant production of heat and loss 

 of movement by frictional motion. He proved that the amoimt of 

 heat produced by friction was continuous, and apparently unlimited ; 

 but he did not think of measuring the relation between the mechanical 

 enerfry expended and the amount of heat produced. Alluding to the 

 results obtained from this apparatus, the lecturer said that Count 

 Eumford might have shown that in his exj)eriments the heat pro- 

 duced was proportional to the time of working, and so obtained 

 a result capable of being expressed in horse-power. The value so 

 deducted from Eumford's experiments is not far removed from Joule's 

 first number. 



The experiments commenced by Count Eumford were carried on 

 by Davy, at that time working with Beddoes at Bristol ; and led to 

 one of the most remarkable essays on heat of that period, which 

 disposed for ever of the theory of the separate existence of caloric. 

 Taking two pieces of ice on a cold day, Davy mounted them so that 

 they could be rotated against each other with frictional pressure, the 

 effect being that the pieces of ice were melted, and the water so 

 produced had a much higher specific heat than the original ice. To 

 guard against the possibility of heat being conveyed to the frictional 



