OF THE MECHANICAL EQUIVALENT OF HEAT. 
369 
in the motion of the mercury, an instrument called G was afterwards employed, each of 
whose divisions was equal to 0°'1911. 
In registering the temperature of the air surrounding the calorimeter it was 
necessary to make allowance for the time wdiich a thermometer takes in altering its 
temperature. I found that in a regularly rising or falling temperature E was 3 in, 8 
behind time, and G 3 m, 127. This lagging of the thermometers was always carefully 
allowed for. 
The capacity for heat of the calorimeter, calculated from the specific heat of brass 
given by Begnault, was equal to that of 5002 grains of water. But Begnault 
has shown how considerably the specific heat of metals of the same chemical 
composition is altered by changes in their hardness, and moreover there were the 
stoppers and other adjuncts to be taken into account. I therefore constructed the 
special apparatus represented by fig. 4, where B, B is a wooden box containing 
the calorimeter h ; the projecting rim of the latter being supported by bits of string 
fastened at the top of three wooden legs, one of which is shown in fig. 5. In 
the lid of the box are three holes which the tubulures of the calorimeter just enter 
without touching. The paddle of the calorimeter can be agitated by means of the 
boxwood piece n. C is a copper vessel covered with a non-conducting substance : 
its lid is perforated to admit a stirrer, a thermometer, and a rod furnished with a 
caoutchouc stopper. 
In experimenting with this apparatus, the calorimeter was first weighed after the 
water which it might have contained was shaken out. It was then placed on its three 
supports, and left for three or more hours in an apartment of uniform temperature, 
until its thermometer ceased to show alteration. The vessel C, containing an adjusted 
quantity of hot distilled water, and placed at some distance, had its gradually 
descending temperature noted from minute to minute. At a given moment it was 
rapidly transferred to the position shown in the figure ; and then on pulling the plug 
out, li was filled in a few seconds. C was then quickly removed, and the caoutchouc 
stopper belonging to the tubulure through which the water had entered having been 
replaced, the temperature of the water was noted again from minute to minute while 
n was constantly moved. These observations afforded the means of eliminating the 
effects of radiation. Finally the calorimeter, as filled with the water, was again 
weighed. 
In the first half of the following Table, A was employed in determining the tem- 
perature of the water introduced into the calorimeter, and D was the thermometer 
plunged into the calorimeter. In the latter half their positions were reversed. The 
temperatures are all given in divisions of A. w includes the estimated value of the 
air displaced, reckoned at 8 grains of water. 
3 e 2 
