OBSERVATIONS IN FOUR BALLOON ASCENTS. 
337 
sented by each series, referring- for further information to the table of observations 
and to the projected results. 
August 17. — We see by the curve of relative humidity for this day, that, from the 
earth’s surface to the height of about 4000 feet, the humidity slightly increased ; the 
presence of a considerable quantity of moisture being also shown by the existence of 
a partial stratum of cloud at the height of about 2500 feet. Between the heights of 
4000 and 5880 the humidity decreased with great rapidity from about 85 to less 
than 35. For a considerable space little alteration took place, with the exception of 
a sudden increase at the height of about 9000 feet, which was confined to a stratum 
of not more than 400 feet ; but as the evidence of its existence depends upon only 
one or two observations it may perhaps be doubtful. From 10,000 feet to 12,300, 
the humidity gradually increased to about 90, which value it retained very constantly 
through fully 4000 feet. After 16,500 feet there were considerable irregularities, 
there being however a comparatively dry stratum between 18,000 and 19,000 feet, 
which was followed by a decided increase in the humidity. These indications agree 
well with what is stated in § 3. with regard to the occasional existence of cloud 
above the height of 13,000 feet, and with the fact that at the highest point reached 
a mass of cloud was seen at a short distance above. In this series we can trace the 
existence of two distinct strata of moist air, besides a third, which undoubtedly 
existed at a greater height, but which was not quite reached. 
August 26, — As on the first ascent, the humidity steadily increased from the earth’s 
surface. Between the heights of 7200 and 8950 feet it also rapidly diminished 
from 92 to 26. For some distance the variations were no greater than might be 
supposed to arise from uncertainty of observation in such extreme circumstances. 
It will be remarked, on examining the curve of the tension of vapour, that whilst the 
indications of Regnault’s hygrometer did not differ much from those of the wet 
thermometer at the height of about 1 1,000 feet, the difference became considerable at 
about 12,000 or 13,000 feet; thus rendering it probable that at the latter heights 
the relative humidity, as deduced from the dry and wet thermometers, was too great. 
The general accordance between the two hygrometers was however nearly restored at 
about 15,000 feet, confirming the rise which there took place in the amount of vapour. 
We may therefore consider that there was little change in the humidity from 9000 to 
14.000 feet, a decided increase having however occurred at 15,000 feet, followed by a 
diminution till 16,400 feet ; an increase having been again indicated in the remainder 
of the curve. The principal stratum of vapour on this day extended from the earth 
to 7200 feet, a second and perhaps a third of smaller thickness existing at 15,000 and 
18.000 feet. 
October 21. — The amount of moisture in the air on this occasion was considerable. 
The relative humidity increased as we left the earth, at first slowly till the height of 
2000 feet, when irregular masses of cloud became frequent, and afterwards with more 
rapidity, till within the principal cloudy mass, at a height of 3450 feet, it attained the 
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