170 THE ANTARCTIC MANUAL. 
not so steady or so strong as on the 19th, and about 5 o’clock in the 
afternoon there was a heavy squall of rain and thunder. The same 
hot Féhn puffs made themselves felt as before, without there being 
any means of measuring their temperature. Their duration at their 
maximum temperature was never more than a few seconds, during 
which but little effect was produced on the thermometer. It occurred 
to me that the only way of gaining a knowledge of the temperature 
of these puffs of air would be by comparing the rapidity with which 
the thermometer moved when exposed to a known difference of 
temperature, with that observed in the puffs. A number of observa- 
tions was made with this view, by warming the thermometer and 
noting its rate of cooling in air of known temperature. The reverse 
procedure was also followed on the ice. The thermometer was cooled 
by being laid close to, but not touching, the ice, it was then quickly 
raised to a height of 1 metre, and its rate of change of temperature 
observed. In this way it was found that for an initial difference of 
4° the thermometer required 10 seconds to rise 1°, for a difference of 
3° 12 seconds, and for a difference of 2°°5 16 seconds. These ratios 
were observed in the open air, and under the circumstances where 
the hot puffs are observed. Unfortunately, owing to an accident to 
the thermometer, very little use could be made of them. Where the 
rate of change of temperature of the thermometer is used to deter- 
mine the temperature of the air, the movement of the air must be 
measured or estimated. The observations made on the 19th and 
21st August are given in Table XXIV. 
“For comparison with the temperatures on the ice on the 19th, the 
mean of the observations at the land station at 2.45 and 4.35 p.m. 
is taken, and on the ice the mean of the observations at 3.20 and 
3.55 p.m. The altitudes of the two stations were as nearly a3 
possible identical, and they were not more than 1 kilometre distant 
from each other. Considering the temperatures at a height of 1m, 
there is a difference of 6°°5 between the land and the ice. The 
difference of vapour tension, 0°2 mm., is insignificant, and shows 
that substantially the air is the same. The dew point in both cases 
is several degrees below 0°, so that, on the air coming in contact with 
the ice, there would be evaporation from the latter. The evaporating 
power of the air may be represented by the difference between the 
tension of saturation and the actual vapour tension. It is very great 
on land, being 10°75 mm. at 16°°53 C., and it would rapidly evaporate 
water having that temperature. On coming in contact, however, with 
ice, the air actually in contact, which alone comes under consideration, 
is first cooled to 0°C., which reduces its saturation tension to 
