144 THE ANTARCTIC MANUAL, 
perature is to be observed. When taken in connection with the 
temperature in the shade this is to furnish information regarding the 
calorific action of the sun’s rays. It is in a position to furnish re- 
liable information of this kind only if the thermal mass or water 
value of the bulb and its term of cooling are known. These are not 
commonly supplied. Even if they were supplied it is doubtful if 
the instrument would furnish any trustworthy information ; the ex- 
ternal glass envelope introduces so much disturbance. 
When a thermometer, whether its bulb be blackened or not, is 
exposed to the direct rays of the sun, its temperature rises at first 
rapidly, then more and more slowly until finally it reaches a tempera- 
ture at or about which it remains steady, provided that it is in a 
place where the air is still and its temperature constant. While its 
temperature remains steady the thermometer is continually receiving 
heat from the sun’s rays and giving it off to its surroundings, and 
the meaning of the constancy of its temperature is that the receipt 
and expenditure of heat per unit of time are equal. 
If we know the thermal mass of the bulb of the thermometer and 
its term of cooling and the difference between its temperature and 
that of the air, we can calculate the rate at which it is losing heat, 
and this, at the stationary temperature, is the rate at which it is 
receiving beat. 
If the thermometer is exposed with its axis perpendicular to the 
direction of the sun’s rays, then it receives the rays which fall ona 
surface equal to the axial sectional area of the bulb. Consequently, 
the heating power of the bundle of solar rays having this sectional 
area is equal to the heating power of the thermometer when the 
excess of its temperature above that of the medium has become 
constant. This is rigorously true of that portion of the pencil 
of rays which penetrates the bulb of the thermometer. If the 
rays have had to traverse the windows of a room, some of the rays 
are absorbed by the glass, and a very large portion is dissipated by 
reflection from it. Further, even a blackened bulb does not transmit 
and absorb all the heat rays that strike it. 
If two thermometers of different pattern are exposed side by side 
to the direct rays of the sun, they usually assume very different 
stationary temperatures, even although their graduation may be per- 
fectly exact. This is to be expected, because thermometers of different 
pattern are sure to differ considerably both in term of cooling and in 
thermal mass. The nearest approach to equality in these particulars 
is found amongst common thermometers of the same pattern, which 
are turned out in large quantities at a time. 
