1921.] The Eighth Indian Science Congress. exi 
ment inside, and the whole in its case. In the front view are 
seen the temperature and humidity arrangements, and the 
three steel pens which draw ona silvered glass plate the records 
of pressure, temperature and humidity. The temperature pen 
is actuated by the differential expansion of brass and invar- 
steel bars, and the humidity pen by the elongation and contrac - 
tion of a sensitised human hair. In the back view appears the 
exhausted aneroid which, by actuating the pressure pen, gives 
a measure of the height reached at every point corresponding 
with the temperature and humidity records. 
31. Each instrument before being sent up has to be cali- 
brated: a very tedious process of going through the whole range 
of pressure and temperature changes likely to be encountered 
and obtaining autographic records of the corresponding res- 
ponses. For pressure calibration the instrument is fitted with 
a clock-driven record plate; and for temperature is cooled in 
spirit by CO, snow in a Dewar’s tube though a range of 
temperatures from that of the air down to —80°C. Fig. 14 
(omitted. 
The humidity calibration is comparatively simple, as satu- 
ration, complete dryness and several intermediate humidities 
are readily obtained in the laboratory. 
Such a calibration is shown in Fig. 15 (omitted) ; it is taken 
on a silyered plate 1 cm. square, and has to be measured up 
under a micrometer microscope. 
Fig. 16 shows the method of sending up the recorder. 
The instrument (a) is suspended in a light bamboo basket 
(b) which is fastened to a parachute (c) of silk, 14 inches square. 
his combination, with a total weight of 45 g. (4 tolas) inclu- 
tinues the temperature of these black spots is kept low by the 
coldness of the air current, but as the ascent slows down the 
sun heats the spots, and they melt and empty the balloon 
