OBSERVATIONS IN FOUR BALLOON ASCENTS. 
313 
tube of the same diameter as that of the barometer. The necessity for this precaution 
was found to be great, as very large differences sometimes existed between the tem- 
perature of the air thermometer and that of the mercury. 
Dry and Wet Thermometers. —Two pairs of dry and wet thermometers were 
employed. One pair was mounted with the bulbs protected from radiation by a 
double conical shade, having highly polished silver surfaces, open at top and bottom 
to allow the circulation of the air. The inner shade was 2 inches high, if inch wide 
at the lower, and half an inch at the upper end : the outer shade was also 2 inches 
^4 inches wide at the lower, and if inch at the upper end*. Both thermo- 
meters were furnished with shades exactly similar ; the bulbs being thus in the same 
circumstances, and completely protected from direct radiation. The thermometers 
were supported, 3^ inches apart, by the arms of a light brass frame, also with a 
polished silver surface. A small brass cistern was fixed near the wet thermometer, 
from which water was conveyed to the bulb by a conducting string of floss silk • 
when however the temperature fell below the freezing-point, the string was cut away 
and the bulb occasionally dipped in water. 
As it was of essential importance that the thermometers should acquire with the 
utmost possible rapidity the temperature of the surrounding air, an arrangement 
was made, in connection with the second pair of dry and wet thermometers, for pro- 
ducing artificially a more rapid current over the bulbs than they would be exposed 
to by the mere vertical motion of the balloon. It was also thought desirable to avoid 
any tendency to a stagnation of the vapour of water in the neighbourhood of the wet 
bulb owing to the want of a sufficient circulation of air to carry it off, as might be the 
case when the balloon was nearly stationary or moving very slowly. An increased 
velocity in the circulation of the air would also tend to remove the effects of radia- 
tion, if the thermometers were not already sufficiently protected by the shades. 
With these objects the following contrivance was adopted. The thermometers were 
fixed vertically with their bulbs enclosed in two tubes placed side by side, and con- 
nected with each other by a cross tube joining their upper ends ; these tubes having 
silver surfaces, and being further protected by a silver shade of the same dimensions 
as the outer shade of the other pair of thermometers. The first tube, in which was 
the bulb of the dry thermometer, had at its lower end a communication with the air : 
by means of an aspirator a current was produced from this opening, upwards over 
the dry bulb, then passing, by the communication at the top, into the second tube 
down which it moved over the wet bulb, leaving it by an opening connected by a 
flexible pipe with the aspirator. By this means, the temperature of the air was deter- 
mined in Its passage over the dry bulb, and afterwards its hygrometric condition on 
coming in contact with the wet ; the vapour of water formed at the latter being ear- 
ned off immediately into the aspirator. The whole distance which the air had to 
* It might have been preferable to make the inner shade cylindrical instead of conical, as the air would have 
circulated more freely. 
2 T 2 
