THE MECHANICAL EQUIVALENT OF HEAT. 
447 
The outside temperature throughout the above experiments was 19°’260 C. The 
actual observation across that temperature was over a range of 0°'128 C. only, and an 
error of 0°‘001 C. in the range would make a difference of 4 in the number in the last 
column. As the rise at this point was one of the most important of all, a straight 
line was drawn through the numbers obtained from the pairs of observations on each 
side of it, the resulting value being •000497. Assuming this to be the rise due to 
stirring only (at rate 30), we get radiation coefficient (p) = (y—•000497)/(l9’260 — t), 
where y is rise (gain) per second due to all non-electrical sources. 
The mean temperature of the first four observations is 14°'475 C., and the mean 
value of y = '853 X 10“'’, hence 
p = -000744. 
The straightness of this “ non-electrical supply line ” is a matter of extreme 
importance, as if there were any marked curvature it would seriously affect our con¬ 
clusions as to the specific heat of water. In the last columns of the above Table we 
give the numbers obtained by direct calculation, assuming p = ’000744 and 
a- = '000497, also the difference between the calculated and experimental results. 
The irregularities are obviously experimental ones, and are to be expected when it is 
remembered that an error of 0°’001 C. in the “range” would account for nearly all of 
them. Again, we are here dealing with our smallest mass of water, and the indi¬ 
vidual experiments are in better agreement when the depth of water is greater. The 
even distribution of the numbers about a straight line is rendered more evident to 
the eye when plotted on a large scale than when presented in rows of figures. The 
Table also illustrates the accuracy of the coiTection for differences in rate of stirring.* 
Assuming the value of the water equivalent of the calorimeter as 85’70, the value 
of M (mass of water + water equivalent) = 103’01 -|- 85‘70 = 188‘71, we get '0140 
as the number of thermal grammes lost or gained by radiation, &c., per second for a 
difference in temperature of 1° C. 
In order to test to what extent any change in the viscosity of the water caused a 
change in the stirring heat, we performed a few experiments with different external 
temperatures. 
* This “ non-electrical snpply curve” liein^ a straight line, we are enabled to deduce the values of 
both p and o- by two experiments only, conducted at any two different temperatures. 
