1921.] The Eighth Indian Science Congress. cil 
the method is the cheaper in operation, the major part of our 
work is carried out by its means. 
The balloons are made of guttapercha tissue, of which a 
sample is on the table. This material was chosen after experi- 
ments had been unsuccessfully tried in Simla with a variety of 
fabrics, and after ready-made rubber balloons from Europe had 
proved unable to withstand the exposure of the journey out. 
The guttapercha is entirely satisfactory: it is light, weighing 
24 grms. per square metre; nearly impervious to hydrogen ; 
readily kept in good condition if stored under water in the 
rk, and is very easily made up into balloons by little-skilled 
labour. 
In practice, such balloons are sent up once daily through- 
Fie. 5. 
out the year at each observing station, and alternate readings 
of azimuth and altitude are recorded at half minute intervals, 
a trajectory, or 
to a known scale. 
the air, without any gui 
In the figure heights in kilometre 
numbers. From the trajectories 
s are marked in non-bracketed 
precise measurements readily 
