DETERMINATION OF THE SPECIFIC HEAT OF WATER. 449 
should be subtracted from our water-equivalents, as there were two such wires. 
The heat produced by the current in the leads (No. 14, I.W.G.) would, if entirely 
used to raise the temperature of the leads, produce a rise of 0°°2 in the time of the 
experiment. If the whole of this heat is imagined to enter the calorimeter it would 
produce a difference of 1 part in 150,000 in the total rise. No error can therefore 
arise through the heating of the leads. 
4. Correction for the parts of the Calorimeter not in contact with Water.—The 
water in our calorimeter reached to about 1°5 centims. from the top of the vessel, 
but a rough estimate of the error introduced will show that we were justified in 
taking the whole of the calorimeter into the water-equivalent. The cooling which took 
place when the temperature of the calorimeter was at 1° above that of the sur- 
roundings amounted to 3°5 gramme-degree units per minute. If we assume that the 
heat lost was the same at every part of the calorimeter, we find that the loss due to 
radiation and convection of the part of the outside surface above the level of the 
water amounted nearly to one quarter-unit. The loss of heat towards the inside of 
the part not filled with water must have been small, but in order to over-estimate 
the loss we may assume that the part of the calorimeter which was above the water 
lost 0°5 heat unit per minute for a difference in temperature of 1°. This will allow us to 
calculate the temperature gradient along the copper close to the water surface. If « is 
the conductivity of the copper, A the area of the section of the calorimeter, ¢ the time, 
KtA du/dx = °5, 
and since « is nearly unity and A = 83, du/dx is found to be nearly ‘01. The 
calculated temperature gradient of +35 of a degree per centim. must have exceeded 
considerably the actual one, and must diminish with the distance from the water 
surface, but assuming it to be uniform all over the upper part of the calorimeter, the 
average temperature of the copper would be 0°:007 below the temperature of the 
water. The heat capacity of that part of the calorimeter is approximately unity, and 
therefore the error in the water-equivalent, considerably over-estimated, is 007, a 
negligible quantity. 
5. Correction for Cork Supports.—It only remains to discuss whether the cork 
supports on which the calorimeter stood added in any appreciable way to the water- 
equivalents. It is not possible here to estimate the effect by calculation, as the 
contact between the calorimeter and the cork is irregularly distributed over the cork. 
We therefore made a few experiments in which the change in temperature of the 
cork was directly measured by means of thermo-junctions, the calorimeter being 
treated exactly as during an experiment on the heat-equivalent. We found in this 
way that the heat which entered the three corks during the whole duration of the 
experiment is only ‘08 unit as against 3300 units which have entered the water in 
the calorimeter, and as the greater part of this heat is allowed for in the cooling 
correction, no addition need be made to the water-equivalents on this account. 
MDCCCXCV.—A. 3M 
