CHEMICAL AND PHYSICAL NOTES. 189 
The agreement is quite satisfactory, and we can therefore use this 
method with perfect confidence in determining the water value of 
the bulbs of all our thermometers. 
The area of the external cylindrical surface is 4°08 x 2°775 = 11°322 cm?. 
Surface of one hemisphere As ce os = 1°225 cm’. 
Whence the total external surface. . . . «. «© . . = 12°547 cm’. 
The area of this surface is the area of the owt/et for heat, and, 
other things being equal, it determines the rate and term of cooling. 
We found the total volume of the bulb to be 2°875 cc., and its water 
value 1°366 grs. It is clear that, ff the area of the external surface 
remains constant and the volume or the water value is increased, 
other things remaining the same, the term of cooling will be increased 
and, of course, the rate diminished. It is quite analogous to the case 
of a water-cistern. If the area of outlet remains the same the time 
which it will take for the cistern to empty itself will depend on the 
quantity of water in it, while, if the size of the cistern and the quan- 
tity of water in it remain the same, the time which it will take to 
empty itself will depend on the smallness of the outlet. Other things 
being equal, the term of cooling of a thermometer will depend on the 
magnitude of the ratio, volume of bulb : area of external surface. 
In this case the ratio is 12°547: 2°875 = 4°364. This may be 
called the virtual cooling radius. In the case of a cooling sphere it is 
the radius of the sphere. 
The physical and thermal data relating to particular thermometers 
have been dweJt upon at great length because they convey a much 
better idea of the nature and propagation of heat than more volumi- 
nous general statements. It may be well here to remind the reader 
that in the department of science which has been called geo-physics, 
when it is a question of specific heat it is nearly always specific heat 
per unit volume that is required, and the thermal study of thermo- 
meters is helpful from this point of view. 
Calorimetrie Constants of a Thermograph—tIt was interesting to 
know what could be obtained with a recording thermometer of 
ordinary type, and in Table XVI. the results of some observations 
made in Cambridge with a Richard’s recorder are given. 
The figures in this table are taken from the curves drawn by the 
instrument on a drum revolving once in forty-eight minutes. The 
instrument was allowed to take the temperature of the room, then 
exposed in the shade in the open air when a fresh breeze was blowing, 
and allowed to remain there until it had taken the temperature of 
the air. It was then transferred to the room, and allowed to rise 
until it attained its temperature. 
