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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
For the first 300 feet was 0°-5 for every 100 feet
From 300 feet to 3400 feet was 0°-4 „ „
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 Shy was partially Cloudy.
In the first 100 feet there was a decline of 0o-9
From 100 feet to 300 „ „ „ 0°-8 for each 100 feet.
„ 300 „ 500 „ „ „ 0°-7
„ 500 „ 900 „ „ „ 0°-6
„ 900 „ 1800 „ „ „ 0°-5
„ 1800 „ 2900 „ „ „ 0°-4
„ 2900 „ 6000 „ „ „ 0°-3
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 10,1.
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 1° 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, thaa that a uniform rate of decrease could be received as a physical law up even to moderate elevations.
§ 6. Variation Of The Htgrometric 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 curvo 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 variation, but during which time there was a great increase of temperature, and consequently the relative humidity decreased with rapidity from 95 to 39. The balloon then fell from 19,500 feet to 19,200 feet, the temperature of the aii- decreased to 38°, and the dew-point increased from 19j° to 21°, and the humidity increased to 49. After 19,200 feet the dew-point decreased with rapidity to 10° at 20,000 feet, with a humidity of 48; and afterwards with great rapidity to a dew-point of less than —12° at 21,000 feet; and at heights exceeding this the dew-point is unknown, but was certainly lower than —20°, and probably as low as —30° up to 24,000 feet; from the observations of the dry- and wet-bulb thermometers it seems to have been as low as —50° at 25,000 feet; therefore the tension of vapour above 20,000 feet must have varied from about 0-015 in. to less than 0-01 in., and the degree of humidity to have decreased to 2, or even less. In this series we can distinctly trace a stratum of moist air in the cloud above 4000 feet, and again between the heights of 9500 feet and 11,500 feet. From 11,500 feet to 19,000 feet the tension of vapour differed but very little from 0-13 inch; then the amount of water present in the same mass of air was nearly constant for 8000 feet in vertical height; immediately after this there were some irregularities, and above 20,000 feet the air was dry, being almost free from vapour.
July 30.—The temperature of the dew-point in this ascent was constantly varying: on the ground it was 53°, at 1000 feet it was 441°, but at intermediate points it was sometimes on one side and sometimes on the other, to the amount of 1° or 2° from the curve-line joining these points; then up to 2400 feet there was a stratum of moist air, and above 3600 feet there were strata of moist and dry air alternately for 2000 feet; higher than this there was a stratum of dry air from 5600 to 6400 feet, and higher still one of moist from 6500 feet to the highest point reachod: these terms, moist and dry, have reference to a curve-line, which was made to pass near every point as laid down from observation; and the same phenomena generally prevailed during the descent. The relative humidity generally increased to the highest point reached.
Table VII. (continued.)
August 18.—The temperature of the dew-point decreased from 57° on the ground to 52J° at 1000 feet, increased from 52£° at 1000 feet to 53L° at L700 feet whilst passing through mist, decreased to 50° at 2000 feet, and varied >ut little till 3800 feet was passed; the degree of humidity varying from 52 on the ground to 96 at 3800 feet when in a cumulus cloud. The dewx>int decreased rather quickly to 41° at 5000 feet, and with less rapidity to 16° at 11,000 feet, the humidity varying from 96 at 3800 feet to 59 at 11,000 feet. Whilst almost stationary in elevation for some time, at the lighest point the temperature of the air increased, whilst that of the dew>oint decreased, so that the degree of humidity changed from 59 to 51. The >alloon then descended: the temperature of the dew-point increased gradually o 31° at 8200 feet, and to 38° at 7800 feet: the humidity was 61 at the ower elevation: the dew-point remained nearly at the temperature of 37° rom 7800 feet to 6000 feet, and rose to 48° at 3500 feet—its lowest
1862. 2 i