PRESENT STATE OF OUR KNOWLEDGE OF THE UPPER ATMOSPHERE. 87 
—60° C. The hygrometer is compared with an aspiration psychrometer 
in an enclosed space. 
This method of testing necessitates somewhat elaborate apparatus, 
and in the German ascents, other than those made at the Lindenberg 
Observatory, the method employed is similar to that of Teisserenc de Bort.: 
The temperature coefficient of the barometer is determined by immersing 
the instrument (the clock having been removed) in the alcohol bath, and 
reducing the temperature. In some cases, instead of alcohol, acetone or 
petrol is employed. Any Bourdon tube or aneroid which has too large 
a temperature coefficient is replaced by a new one. The temperature 
correction of the barometer is applied in the form proposed by Hergesell 
and Kleinschmidt,” viz. :-— 
Ap=-—AT(A—ayp), 
where A p is the pressure correction to be found, 
A T the fall of temperature, 
A, a constant varying with different instruments, 
a, a constant for a given type of instrument, 
p, the pressure uncorrected. 
The constant a, according to Hergesell and Kleinschmidt, is 0°00046 
for the Bosch-Bourdon tube and 6*°00064 for that of Richard. 
For a typical Bosch-Hergesell instrument the correction was given 
by the equation :— 
Sp = — AT (048 — 0:00046 p). 
In Germany, Kleinschmidt also tests the hygrometers at different tem- 
peratures. He found * that the length of the hairs was independent of 
the pressure and almost independent of the velocity of the ventilating 
air-current. The hygrometer was affected little by variations of tem- 
perature between +20°C. and +5°C., but became very sluggish below 
—10°C. It recorded slow variations fairly well at —30° C., but was not 
even qualitatively suitable at — 40° C. 
Test of Dines’ Ballon-sonde Barograph. 
Temperature Deflection | Pressure Deflection 
°C”. mm. | mm. mm 
15 +0°30 760 0°60 
| 600 3°35 
300 10 78 
50 15°85 
0 0:00 760 0:0 
600 2°76 
300 10:00 
50 15°28 
— 30 — 0°60 760 —1-20 
600 +1:58 
300 8°72 
| 50 14 75 
—60 —1:20 760 — 2°30 
600 +0°38 
300 7:58 
50 14:24 
t 
’ At Munich, Schmauss places the instrument in an air chamber surrounded by 
an alcohol bath. The air is thoroughly mixed by means of two fans. 
2 Beitrage z. Physik der Freien Atmosphdare, 1, 1905, pp. 108-119, 208-210, 
3 Thid, 2; 1906-07, p. 99. 
