BALLOON ASCENTS. 
415 
The first instraments used for this investigation were ther- 
mometers as delicate as those screened from the sun^ usually 
located near to the dry-bulb thermometer^ but readily re- 
moved to any other position within the sun^s influence. It was 
soon found that the excess of readings was very sinall ; and 
at the height of five miles high_, with a brighter sun than on 
the earth, the difference, if any, was very small indeed ; at 
the heights of 4 miles, 3 miles, 2 miles, the difference became 
larger, and increased on approaching the earth. 
Having advanced thus far in the inquiry, with results differ- 
ing a good deal from those expected, it became a matter of 
considerable importance in reference to our knowledge in this 
respect, and as possibly leading to some information which 
might tend to some quality in the heat-rays, to confirm these 
results by the use of other instruments ; for it was just possible 
that the small projecting bulb of a thermometer, kept free 
from all objects to reflect or conduct heat to it, might receive 
them from the sun, and by the quick and constant motion of 
the balloon, might at once part with them again to the cold air 
in immediate contact with the bulb. Another and similar ther- 
mometer, enclosed in a vacuum tube, was used, the bulb being 
surrounded by a large glass globe, admitting the heat to pass 
directly to the bulb, but parting with none to the surrounding 
air ; the results by the use of this instrument seemed to con- 
firm those previously found. 
These researches led to the use of Herschehs actinometer, 
shown at the extreme right hand in the plate. It is an 
awkward instrument in a balloon, and somewhat difficult to 
use, but the views then opening to me, and of which I shall 
presently speak, indicating a new link in the chain of our 
knowledge by which the several members of our solar system 
are united together, by receiving heat from the sun, in pre- 
cisely the same manner, and possibly to the same amount, 
were so important, and this instrument was the only one I 
knew which could give the necessary information, I resolved 
to contend with the difficulties and use it in the balloon. The 
general results from its use are as follows : — That, when on 
the ground, the number of scale division^ increasing in a 
minute were between 40 and 50 j at the height of three miles, 
with a deeper blue sky, and a brighter sun, the increase in 
the same time will be but seven or eight divisions, agreeing 
with the previous series of experiments. 
These remarkable results lead us to new ideas respecting 
the passage of the heat-rays through space. From them, it 
would seem as highly probable that the heat-rays from the 
sun pass through space without loss, and become effective 
where wanted only, and in proportion to the density of the 
