HIRN'S VERIFICATION. 153 



the mechanical equivalent of heat by the friction of 

 water ; introducing a great simplification in the measure- 

 ment of the work expended. This was accomplished by 

 Hirn's recognition of the principle that the moment of 

 resistance which the water opposes to turning of the 

 paddle must be equal to the moment of resistance which 

 the containing vessel opposes to the water, so that instead 

 of measuring the force spent on the moving paddle it was 

 only necessary to measure the force to keep the can from 

 turning. Hirn also succeeded in verifying, for the first time, 

 Joule's views expressed in 1847 as to the conversion of 

 lieat into work in the steam engine, as shown by the 

 difference between the heat received from the boiler and 

 discharged by the condenser, thus instituting that system 

 of steam engine trials which now govern all practice. At 

 this time, too, the works of John Tyndall and of Balfour 

 Stewart had gone far to render Joule's views popular and 

 introduce them into the curricula of school education. 



