OF ARTS AND SCIENCES. 1(3 



To see if the amount was appreciable, I suddenly threw the appa- 

 ratus out of gear, thus stopping it. The temperature was observed to 

 continue rising about 0°.02 C. Allowing 0°.01 for the rise due to 

 motion after the word "Stop" was given, we have about 0°.01 C. as 

 the amount the thermometer lagged behind the water. 



4. Evaporation. A possible source of error exists in the cooling of 

 the calorimeter by evaporation of water leaking out from it. 



The water was always weighed before and after the experiment in 

 a balance giving ^ 6 gramme with accuracy. The normal amount of 

 loss from removal of thermometer, wet corks, &c. was about 1 

 gramme. The calorimeter was perfectly tight, and had no leakage at 

 any point in its normal state. Once or twice the screws of the stuffing- 

 box worked loose, but these experiments were rejected. 



The evaporation of 1 gramme of water requires ahout GOO heat 

 units, which is sufficient to depress the temperature of the calorimeter 

 about 0°.07 C. As the only point at which evaporation could take 

 place was through a hole less than l mm - diameter in the safety-tube, I 

 think it is reasonable to assume that the error from this source is in- 

 appreciable. But to be doubly certain, I observed the time which 

 drops of water of known weight and area, placed on the warm calo- 

 rimeter, took to dry. From these experiments it was evident that it 

 would require a considerable area of wet surface to produce an ap- 

 preciable effect. This wet surface never existed unless the calo- 

 rimeter was wet by dew deposited on the cool surface. To guard 

 against this error, the calorimeter was never cooled so low that dew 

 formed ; it was carefully rubbed with a towel, and placed in the appa- 

 ratus half an hour to an hour before the experiment, exposed freely to 

 the air. The surface being polished, the slightest deposit of dew was 

 readily visible. The greatest care was taken to guard against this 

 source of error, and I think the experiment is free from it. 



(d.) Results. 

 1. Constant Data. 



Joule's equivalent in gravitation measure is of the dimensions of 

 length only, being the height which water would have to fall to be 

 heated one degree. Or let water flow downward with uniform velocity 

 through a capillary tube impervious to heat ; assuming the viscosity 

 constant, the rate of variation of height with temperature will be 

 Joule's equivalent. 



Hence, besides the force of gravity the only thing required in ab- 



