442 PROFESSOR A. SCHUSTER AND MR, W. GANNON ON A 
The reason for the last item will be given in connexion with the discussion of the 
cooling correction. 
The Total Mass of Water. 
The calorimeter contained approximately 1514 grams. of distilled water in each 
experiment. The mass of water was determined by volume and by weight. In the 
estimation by volume a carefully calibrated 500 cub. centims. flask and burette were 
used. The weighings were taken on a balance, allowing, to estimate accurately, 
0°1 gram. The buoyancy correction amounts to 1°58 gram. 
As our calorimeter was not completely covered, a certain amount of evaporation 
took place, the mass of water during the experiment differing by about a decigram 
from that which was put in. <A few experiments give a fairly consistent rate of 11 
gram. per hour of evaporation under the conditions of our experiment. As a check 
the water was nearly always weighed both before and after each experiment, and the 
mass during the experiment was then found by interpolation. 
The Cooling Correction. 
The interchange of heat between the calorimeter and its surroundings depends on 
conduction and radiation. If the water of the calorimeter is exposed to the 
air, evaporation may sensibly lower the temperature, but for the present we may dis- 
regard the effects of evaporation. The loss of heat to the outside is usually corrected 
for in a well-known manner. The correction depends on the assumption that the loss 
of heat is proportional to the difference in temperature between the calorimeter and 
its enclosure, and if the loss of heat is small, the assumption is generally justified. In 
a well-disposed experiment the temperature (w) of the calorimeter alters slowly and 
regularly during the preliminary period. If ” observations are made at regular 
intervals of time T, they should be expressible with sufficient accuracy in an expression 
of the form u=v-+ kT. It is usual to obtain the two constants of this equation v 
and k by graphical means; we have employed instead the method of least squares, 
not for the reason that we believe to have obtained a greater degree of accuracy in 
this way, but simply as a matter of convenience. ‘The process of calculation takes 
less time than the graphical method, and the results are quite free from personal bias. 
The observations themselves give a series of equations, 
Uy =v+kT 
Uy = 0 + 2kT 
wy, =v + nkT. 
It is required in the first place to calculate v and &. The most probable values of 
v and k are 

