122 



HEAT. 



noted, and the effect of radiation to or from the surroundings deter- 

 mined by subsidiary experiments. Knowing the capacity for heat 

 of the calorimeter, the total quantity of heat generated could be 

 determined. 



The result obtained was that the kinetic energy due to the fall of 

 772 Ibs. through 1 foot at Manchester would, on transformation to heat, 

 raise 1 Ib. of water between 55 and 60 F. through 1 F. 



Joule also made experiments with mercury instead of water, using an 

 iron calorimeter, and he obtained as a result 774. 



In another series of experiments, he made the masses, in falling, turn 



Horizontal* Section of 

 Calorimeter. 



Vertical Section, of 

 Calorimeter 



FlG. 75. Joule's Water-Churning Apparatus for Determining the 

 Mechanical Equivalent of Heat. 



a bevelled iron wheel against another fixed bevelled iron wheel, both 

 being placed in the calorimeter and surrounded by mercury. The result 

 was nearly 775. It is worthy of note, as illustrating the care with 

 which he experimented, that he allowed for the loss of energy by the 

 sound given out by the vibration of the apparatus, estimating it by 

 the work required to produce an equal sound as heard at the same 

 distance from a violoncello. But neither of the modes of experiment 

 with mercury was quite as satisfactory as that with the water calori- 

 meter, and they were disregarded in favour of the water-friction 

 experiment. 



Later Repetition. Many years after the publication of Joule's paper 

 giving an account of this work, the British Association, in framing a 



