Meteorology in its connection with agriculture / by Joseph Henry.

  • Henry, Joseph, 1797-1878.
Date:
1858
Catalogue details

Licence: Public Domain Mark

Credit: Meteorology in its connection with agriculture / by Joseph Henry. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    16/46 (page 466)
    even of low intensities, in this region, that a very remarkable differ- ence was observed on some occasions when the camp-ground was chosen in a gorge between two steep hills. The inter-radiation between the hills prevented in a measure the usual diminution of temperature, and the thermometer in such a position stood several degrees higher than on the open plain. We shall next briefly consider the mechanical constitution of the atmosphere. The aerial ocean which surrounds the earth consists of atoms of matter self-repellent, which, in proportion as the inte- rior pressure is lessened, constantly tend to separate from each other, and produce an enlargement or expansion of the whole mass. When the pressure is increased, the mass sinks into a less volume, the atoms are brought nearer together, the force of repulsion is increased with the diminution of distance between the atoms, and a new equilibrium is attained. From this constitution of the air, it immediately follows that the density of the atmosphere near the surface of the earth is greater than that at a higher altitude, since the lower stratum bears the weight of all those which are above it. The diminution in weight of equal bulks of air as we ascend is in a greater ratio than the height, since it diminishes on two accounts: first, because as we ascend in the air the number of strata pressing on us is less; and secondly, each succeeding stratum is lighter. From the law of this diminution of density a table may be formed of the pressure of the atmosphere at various heights, of which the following is an example: Density of the air at increasing altitudes Miles above the sea. Bulk of equal weight of air. Density. Height of barometer. 0 1 1 30.00 3.4 2 15. 00 6. 8 4 \ 7.60 10. 2 8 i 3. 75 13. 6 16 v'b 1. 87 17.0 32 A 0. 93 From this table it appears that one-half of the whole atmosphere is found within the limit of height of 3| miles, and one-third of the whole quantity beneath the level of the Kocky Mountains ; and this fact has an important bearing on the influence of mountain ranges in modifying the direction of the winds. The question occurs at this place. Why does the air grow colder as we ascend ? The answer is, that a pound of air, at all distances above the earth, contains at least an equal amount of heat with the same weight taken at the surface, and that, as the pressure is re- moved, this air is expanded in bulk, and consequently the heat is diffused through a greater amount of space; and hence the reduc-