71 



first table, we find 185" 7 for the amount of work due to a unit of 

 heat descending from a body A at 100° to B at 0°. Now 79 such 

 units of heat would, according to Dr. Black (his result being very 

 slightly corrected by Regnault), melt a kilogramme of ice. Hence 

 if the heat necessary to melt a pound of ice be now taken as unity, 

 and if a metre-pound be taken as the unit of mechanical effect, the 

 amount of work to be obtained by the descent of a unit of heat from 

 100° to 0° is 79 x 135-7, or 10,700 nearly. This is the same as 

 35,100 foot-pounds, which is a little more than the work of a 

 one-horse-power engine (33,000 foot-pounds) in a minute ; and 

 consequently, if we had a steam-engine working with perfect ceco- 

 nomy at one-horse-power, the boiler being at the temperature 100°, 

 and the condenser kept at 0° by a constant supply of ice, rather 

 less than a pound of ice would be melted in a minute. 



