The next Table has been formed by taking the difference between the temperatures at every consecutive 100 feet, in every ascent and descent, up to 5000 feet.

TABLE VI.-Showing the Decrease of Temperature with every

Ascending.

From

To

Ascending.

Descending.

An inspection of this Table shows that the largest numbers are those situated at the bottom, and the smallest at the top of each column in all states of the sky, and therefore that the decline of temperature in equal spaces was largest in that space next the earth, and gradually less with increase of elevation.

- The numbers in the last column of the Table show the average value at each 100 feet, the one in cloudy states of the sky, and the other in partially clear states, with the number of experiments upon which each result is based.

FROM THESE RESULTS THE DECLINE OF TEMPERATURE

When the Sky was Cloudy

For the first 300 feet was .....
.....0°.5 for every 100 feet.
From 300 feet to 3400 feet was 0°.4

3400

0°.3

Therefore in cloudy states of the sky the temperature of the air decreases nearly uniformly with the height above the surface of the earth nearly up to the cloud.

When the Sky was partially Cloudy.

In the first ......100 feet there was a decline of 0°.9
From 100 feet to 300

0°.8 for each 100 feet.

The decline of temperature near the earth with a partially clear sky is nearly double of that with a cloudy sky; at elevations above 4000 feet, the changes for 100 feet seem to be the same in both states of the sky.

In some cases, as on July 30, the decline of temperature in the first 100 feet was as large as 1°.1.

From these results we may conclude that in a cloudy state of the sky the decline of temperature is nearly uniform up to the clouds; that with a clear sky the greatest change is near the earth, being a decline of 1o in less than 100 feet, gradually decreasing, as in the general law indicated in the preceding Section, till it requires a space of 300 feet at the height of 5000 feet for a change of 1° of temperature. These results lead to the same conclusion as before, viz. that the theory of gradation of 1° of temperature for every 300 feet of elevation must be abandoned. As regards the law indicated by all these experiments, it is far more natural and consistent, than that a uniform rate of decrease could be received as a physical law up even to moderate elevations.

§ 6. VARIATION OF THE HYGROMETRIC CONDITION OF THE AIR WITH ELEVATION. All the adopted readings of the temperature of the dew-point in Section 4 were laid down on diagrams of a large scale, and their points were joined; and as it was evident that there were strata of moist air, and that the changes do not follow any regular decrease as in the case of the temperature of the air, it was therefore not considered prudent to adopt any curve with the view of obtaining normal results, but to use the projected curve as simply found by joining the points as stated above. From the readings at every 1000 feet of

elevation the next Table was formed; other readings were taken at angular intermediate points, and these are included in the remarks following the Table. The numbers under the heading of "Tension of Vapour" are formed by using "Regnault's Tables," and the degree of humidity in the next column has been calculated by using the observed temperature of the air corresponding to the observed temperature of the dew-point.

TABLE VII.-Showing the Variation of the Hygrometric condition of the Air at every 1000 feet of Height.

July 17.-At the earth's surface the dew-point was 55°, which seemed to decrease gradually to the height of about 4000 feet, the relative humidity decreasing from 79 to 65 within the same space; on entering a cloud the rate of the decrease of the dew-point was checked, and for a space of 3000 feet was almost constant, differing but little from 32°, whilst the relative humidity increased to 91 at 5800 feet. On leaving the cloud at 8000 feet high, and between that and 9600 feet, both the dew-point and the relative humidity decreased quickly, the former to 170.9 and the latter to 65. From 9600 feet to 11,500 feet, whilst the temperature of the air remained at 26°, the dewpoint increased to 24°.8, and the relative humidity to 95, closely approaching to saturation. From the height of 12,000 feet to 19,000 feet, the amount of water in the air was almost constant, the dew-point undergoing scarcely any