OBSERVATIONS IN FOUR BALLOON ASCENTS. 
315 
velocity as the ascent, and observations were continued to within 3000 feet of the 
earth. The differences of temperature at the same height are scarcely appreciable, 
and even frequently in the opposite direction from what would result from insensibi- 
lity in the thermometer. A few observations were taken during the descent on 
August 17, which, when compared with those made at the same height in the ascent, 
show a difference of about four degrees. The rapidity of the descent was on that 
occasion about twice as great as that of the ascent, which was also considerable, and 
the thermometers were not under aspiration. The protection from radiation has 
been examined by observing the thermometers within a room when alternately ex- 
posed to strong sunshine and shade — the effect upon the aspirated thermometers 
did not exceed l °-5 : in the open air, and with a gentle breeze, the effect was 
considerably less. The effect upon the free thermometer appeared to be greater ; 
and the difference between its indications and those of the aspirated thermometer 
during some of the ascents is probably to be ascribed to this cause. It would appear 
from some portions of the series of August 26 , that the long-continued exposure to 
the sun, in a nearly calm air, has produced an appreciable effect upon the readings 
of the thermometer, whether aspirated or free. Fortunately, with the exception of 
the ascent of August 26 , the sun’s radiation was never powerful ; whilst on August 17, 
when the free thermometers were alone observed, the sun was scarcely ever visible. 
When the radiation was feeble, and the vertical motion of the balloon considerable, 
the two thermometers agreed very closely*. As hygrometers there is probably less 
difference in their value. In the examination of the results of the temperature ob- 
servations, I have been led to prefer the indications of the aspirated thermometer. 
Regnault’s Condensing Hygrometer \. — The only difference in the construction of 
this instrument, from that usually adopted by M. Regnault, was that the small tube, 
by which the air enters the reservoir to agitate the ether, had a funnel-shaped 
opening at top to facilitate the supply of ether. The bulb of the thermometer was 
cylindrical, fi inch long, and i^th of an inch diameter. The scale was of ivory, and 
the thermometer was fixed into the reservoir by a cork. 
No use was made of Daniell’s hygrometer as that of M. Regnault was found much 
more convenient for such observations, being to a great extent self-acting. 
Polariscope. — This instrument was supplied by Mr. Darker of Lambeth. Its 
principle is the same as that employed in Mr. Wheatstone’s ‘‘ Polar Clock the 
parts of the polariscope used having, I believe, actually formed portions of one of 
those instruments. It exhibits the existence of polarization in a conspicuous 
manner. 
Exhausted Tubes for Collecting Air. — These tubes, which were constructed by 
Messrs. Negretti and Zambra, were about 9 inches long and f inch diameter, fitted 
* It would be advisable in any future experiments to apply additional shades to all the thermometers, and if 
possible to use a larger general screen at some distance from them. 
t This instrument is described in the “ Annuaire Meteorologique de la France” for 1849, p. 221. 
