OF ARTS AND SCIENCES. 137 



ever, the difference could not at most amount to more than 0.5 per 

 cent, which is very satisfactory. 



The total capacity of the calorimeter is reckoned as follows: — 



Weight of calorimeter 3.8712 kilogrammes. 



" screws .0016 



" part of suspending wires .0052 . " 



Total weight 3.8780 " 



Capacity = 3.878 X -0892 = .3459 kilogrammes. 



To this must he added the capacity of the thermometer bulb and sev- 

 eral inches of the stem, and of a tube used as a safety valve, and we 

 must subtract the capacity of a part of the shaft which was joined to 

 the shaft turning the paddles. Hence, 



.3459 

 + .0011 

 -(- .0010 

 — .0010 



Capacity = .3470 



As this is only about four per cent of the total capacity, it is not 

 necessary to consider the variation of this quantity with the tempera- 

 ture throuidi the ransre from 0° to 40° which I have used. 



IV. DETERMINATION OF EQUIVALENT, 

 (a.) Historical Remarks. 



The history of the determination of the mechanical equivalent of 

 heat is that of thermodynamics, and as such it is impossible to give 

 it at length here. 



I shall simply refer to the few experiments which a priori seem to 

 possess the greatest value, and which have been made rather for the 

 determination of the quantity than for the illustration of a method, 

 and shall criticise them to the best of my ability, to find, if possible, 

 the cause of the great discrepancies. 



1. General Review of Methods. 



Whenever heat and mechanical energy are converted the one into 

 the other, we are able by measuring the amounts of each to obtain 

 the ratio. Every equation of thermodynamics proper is an equation 



