July 17 10 8 14544 240 26.7 20.5 240-27 +35

Between 15,000 and 16,000 feet.

0.0 +62 +2°7-3'5

July 17 10 11 15704 246 22.7 220 215 +19 +2.6+3°3 +0.7 +14 +0.7

Between 16,000 and 17,000 feet.

July 17 10 15 16914 24°3 19.8 230 208 +45 +13 +35-32-10 +2.2 Aug. 18 2 31

....

-0°7

6'0

..

..

..

-6'7

By taking the mean of each column of differences in each 1000 feet of elevation, the next Table is formed.

TABLE X.-Showing the mean differences between the Temperatures of the Dew-point as found by the use of the Dry- and Wet-bulb Thermometers and by Daniell's and Regnault's Hygrometers, and comparison of the results as found from the two Hygrometers.

The numbers in every one of these columns are affected with a change of sign, and, therefore, no certain difference is shown over the determination of the dew-point as found by any method or instrument over that found by any other.

By taking the means of all in each group of 5000 feet, giving weight according to the number of experiments upon which each result is based, we have:

From the ground to 5000 feet high the temperature of the dew-point as determined by

Dry and Wet bulb (free)

Experiments.

Was 0°-4 higher than as found by Dry and Wet (aspirated)..from 4 the same as found by Daniell's Hygrometer

20.2 higher than as found by Regnault's Hygrometer

73

Dry and Wet bulb (aspirated)

Was 0.1 higher than as found by Daniell's Hygrometer

1o.0

25

From 5000 feet to 10,000 feet the temperature of the dew-point as determined by

Dry and Wet (free)

Was 0°.2 higher than as found by Dry and Wet (aspirated) ..from 1

Daniell's Hygrometer

From 10,000 to 15,000 feet the temperature of the dew-point as determined by

Dry and Wet (free)

Was 20-1 lower than as found by Dry and Wet (aspirated) . . from 3

Was 6°.2 higher than as found by Daniell's Hygrometer

From 15,000 to 20,000 feet the temperature of the dew-point as determined by

Dry and Wet (free)

Experiments.

Was 10-1 higher than as found by Dry and Wet (aspirated)..from 7

Was 0°-6 lower than as found by Daniell's Hygrometer...

.....

Daniell's Hygrometer

Was 0.4 higher than as found by Regnault's Hygrometer....,,

8

By taking the mean of all, according to the number of experiments, we have:

:

From the ground to 20,000 feet the mean temperature of the dew-point as found by

Dry and Wet (free)

Was 0.2 higher than as found by Dry and Wet (aspirated) ..from 15

Was 0°-9 higher than as found by Daniell's Hygrometer

....

22

Was 0.1 lower than as found by Regnault's Hygrometer

From all the results it would seem that the temperature of the dew-point as deduced from the Dry- and Wet-bulb thermometers as ordinarily used has a tendency to give a result a little too high, but to an amount that is less than the probable error of observations, and that, therefore, it is a perfectly trustworthy instrument to use, even to great altitudes; also, the results by Daniell's Hygrometer seem to be of equal value with those found by Regnault's Hygrometer, at all elevations.

§ 8. COMPARISON OF THE READINGS OF THE MERCURIAL AND ANEROID BAROMETERS AT DIFFERENT HEIGHTS.

All the simultaneous readings of the Siphon and Aneroid Barometers were extracted from Table I. and inserted in the following Table.