100 



THE FARMER'S MAGAZINE. 



since had shown that the amount of steam existing 

 in a given space and temperature is the same, 

 whether that space be free from or filled with air ; 

 and then Dalton distinctly proved that the vapour 

 of water mixed with air, or other gas permanent over 

 water, diflFers in no respect from pure steam, and 

 is subject to the same laws. The aqueous vapour 

 of the air constitutes, in his opinion, a distinct and 

 mdependent atmosphere, the elastic force of which 

 forms at different temperatures different propor- 

 tions of the elastic force of the whole. For 

 example, at the temperature of 65 deg. it gives to 

 air l-50th of its elasticity. According, therefore, 

 to this view, which is confirmed by the experiments 

 of Gay Lussac and others, a volume of air, or 

 gas, at any temperature, saturated with moisture, 

 contains as much steam as would exist, at the same 

 temperature, in a vacuum of the same extent. 



The insensible vapour in the air we may then 

 conclude is merely mechanically mixed with the 

 atmospheric gases; there is no chemical combina- 

 tion. It is the diffusion of water in the state of 

 steam, produced by the evaporation from the 

 earth's surface. This evaporation is hardly ever in- 

 terrupted ; it continues very often even when rain 

 is falling, or the ground covered with snow — under 

 the burning sun of the equator, or in the eternal ice 

 of the arctic regions, it still proceeds. It is, in- 

 deed, remarkable, as the Rev. L. Jenyns observes 

 (" Meteorology," p. ] 64), that evaporation still 

 goes on when water is frozen, the same as when it 

 is liquid : even the most intense cold is insufficient 

 of itself to put a stop to it. This circumstance 

 often strikes persons with astonishment who wit- 

 ness it in its effects without being aware of the 

 true cause. They see a fall of snow gradually 

 waste — if light, wholly disappear — or a block of ice 

 sensibly diminish during the continuance of a 

 frost, especially if the wind blows tolerably fresh 

 from some point towards the nortli, without the 

 least sign of liquefaction on the surface. And 

 they perhaps naturally enough wonder what has 

 become of it. Sometimes also in deeper snows the 

 surface becomes curiously grooved or channelled, 

 by the wind acting unequally on it, and thus pro- 

 moting unequally the evaporation. This pheno- 

 menon is best observed around the trunks of trees, 

 and near the interstices of palings, or wherever a 

 stream of air acquires an increased force in a par- 

 ticular direction. There is every reason to con- 

 clude with Dr. Prout, that the quantity of vapour 

 thus formed from snow and ice is precisely equal to 

 what would be evaporated from water itself, pro- 

 vided water could exist as a fluid below the tem- 

 perature at which it is congealed. 



The amount of water in the air, from an average 

 of seven years' observation at the Greenwich Ob- 

 servatory during each month of the year, has been 

 given by Mr. J. H. Belville, in his "Manual of the 

 Thermometer." 



In the following table, column I. gives the mean 

 weight, in grains, of vapour in a cubic foot of air at 

 9 o'clock a.m., and column II. the amount at 

 3 o'clock p.m. Column III. gives the mean addi- 

 tion of vapour required for complete saturation of 

 a cubic foot of air at 9 o'clock, and column IV. 

 the amount needed at 3 o'clock. 



The amount of water which the air contains, let 

 us remember, increase's with its temperature. The 

 mean relative humidity of the air, Mr. Belville ob- 

 serves, is greater at 9 a.m. than at 3 p.m.; the 

 mean quantity of vapour in this time actually 

 increases, but as the increase is not in proportion 

 to the increase of temperature in the same interval, 

 the air is relatively drier. 



November, December, and January are the 

 months when the air is most frequently satu- 

 rated with vapour; as spring advances the air 

 becomes warmer, and the point of saturation far- 

 ther removed. 



A cubic foot of atmospheric air, when saturated 

 with water at the temperature of 66 degrees, con- 

 tains only about eight grains of water. Dalton 

 calculated that the medium quantity of vapour held 

 in solution at once in the atmosphere may amount 

 to about one-seventieth of its bulk. 



That vapour we have seen is mainly supplied by 

 the evaporation of the surface of the ocean ; but the 

 land contributes a large proportion ; the vegetable 

 and animal worlds do the same. First as to the 

 portion yielded by the land, the amount of water 

 evaporated from its surface has been examined by 

 various experimentalists. Schubler conducted his 

 trials on the small scale, with different earths ex- 

 posed in trays to the sun and winds {Journal R. 

 A. S., vol. i. p. 177). He found that the difference 

 in the evaporation from the surface of different 

 earths was not so great as might have been antici- 

 pated. He determined that when a given surface 

 of calcareous sand lost by evaporation during four 

 hours 146 parts, an equal extent of fine garden- 

 mould lost 143 parts, some black turf soil 128 

 parts, and two specimens of clay soil each lost 

 123 parts. 



Some years after Schubler's experiments, Mr. 

 Dickenson {ibid. vol. v. p. 151) examined the 

 amount of the annual evaporation from the chalk 

 soils of Hertfordshire. The following were some 

 of the results he obtained in 1842 and 1843: — 



1842. 1843. 



Depth of rain in inches 26.43 26.47 



Evaporation 14.7 18.37 



Filtration 11.76 8.10 



In the same years, on the limestone soils of 

 Yorkshire, Mr. C. Charnock obtained the follow- 

 ing results {ibid., vol. x., p. 517) : — 



