134 THE ANTARCTIC MANUAL. 
the thermal mass of the bulb of the thermometer and the method 
of determining it. When we have these two constants, namely, the 
term of cooling and the thermal mass, the thermometer becomes a 
calorimeter, and its radius of research is much increased. 
The thermal mass of the bulb of a thermometer is completely 
specified if we know (a) the weight of mercury which it contains; 
(6) the weight of the glass which forms the envelope; (c) the specific 
heat of mercury ; and (d) the specific heat of the particular glass used. 
As the mercury used for thermometers must be perfectly pure, ¢ is 
known. Many different kinds of glass are used, and the information 
regarding their specific heat is very defective. This constant should 
therefore be determined for the particular sample of glass used in 
the construction of the thermometers. By working in association 
with the thermometer maker there is no difficulty in ascertaining the 
exact weight of mercury in the thermometer, or that of the glass 
which goes to the bulb. Then if we multiply the weights of these 
substances used by their respective specific heats, the sum of the 
products is the thermal mass of the bulb expressed as the weight, in 
grammes, of water, which is thermally equivalent to it. This constant 
is usually, and conveniently, called the water value. Calorimetry is 
an important department of physics and physical chemistry, and the 
methods of determining water values are given in all treatises on 
the subject. Where the size of the thermometer is considerable 
the specific heat of its bulb can be determined directly by the 
old “method of mixtures,” the thermometer itself being one party 
to the mixture. As the term of cooling of a thermometer increases 
in proportion to the size of the bulb, it is clear that thermo- 
meters intended for meteorological use should have as small bulbs as 
possible, and the method of mixtures is not applicable for the 
determination of their thermal masses. Again, it is out of the 
question to expect these thermometers to be constructed so that the 
respective weights of mercury and of glass in their bulbs shall be 
accurately known. The following method of determining this con- 
stant is very convenient. It depends on mensuration, and was 
published by the writer in 1894.* 
Estimation of the Thermal Mass of the Bulb of a Thermometer by 
Mensuration.—The method is shortly stated in the following Rule: 
Determine the external volume or displacement of the bulb in cubie centi- 
metres ; multiply it by 0°475, and the product is the water value of the 
* «On Rapid Variations of Atmospheric Temperature, especially during Féhn, and 
the re of observing them,’ by J. Y. Buchanan, F.B.S. Proc. R. S. (1894), vol. lvi. 
p. 126. 
