142 THE ANTARCTIC MANUAL. 
capacity for heat per unit of weight are higher than in the case of 
ordinary German glass, but the specific heat per unit of volume is 
probably very little affected. 
Calorimetric Constants of Deep-sea Thermometers.—It is evident 
that a knowledge of the calorimetric constants of the deep-sea 
thermometer is necessary, if we are to have the conviction that the 
temperature indicated by it is in truth the temperature of the water 
in which it was immersed. This is all the more necessary because, 
in order to guard the bulb of the thermometer against the squeezing 
effect of the pressure of the column of water to which it is exposed 
when in use, it is hermetically enclosed in an outside bulb, the 
space between them being partially filled with mercury. This extra 
bulb increases greatly the term of cooling of the thermometer. 
The conditions are quite analogous to those regulating the cooling 
of thermometers in air. The term is comparatively long when the 
thermometer is immersed in still water and kept motionless in it. 
When the water has relative motion with regard to the thermometer 
the term is reduced in proportion to that motion. 
For practical purposes we require to know how long we must 
leave the thermometer at the particular depth in order to be sure 
that it has taken the temperature of the water. The experiments re- 
quired in order to furnish this knowledge are extremely simple. The 
principle is exactly the same as that which governs the behaviour 
of thermometers in air. The thermometer loses equal fractions 
of its excessive heat in equal intervals of time. These intervals are 
very much shorter when the instrument is immersed in water than 
when it is in air. When the difference of temperature is at all con- 
siderable the thermometer falls very rapidly at first, and more slowly 
as it approaches to the temperature of the water. The divisions of 
the scale of a thermometer ought to be about one millimetre apart. 
If in reading the thermometer we estimate tenths of this amount, 
then it is important to know how long the thermometer takes to 
assume the temperature of the water within one-tenth of one of its 
own divisions. If we estimate only to one-half of a division, then it 
is sufficient to know how long the thermometer takes to arrive at that 
of the water within one-half of one of its own divisions, It does not 
matter what the thermometric value of each division is. The follow- 
ing imaginary case will illustrate this. It is assumed that when 
immersed in still water the thermometer loses half its excessive heat 
in twenty seconds. When immersed in the water the excessive 
temperature of the thermometer is represented by 4°8 of its own 
divisions. 
