598 



HYGROMETRY. 



in the stra- 

 ta of the 

 atmosphe- 

 rical co- 

 lumns, at 

 various 

 heights. 



Mrgromc. It appears by this Table, that if the atmospherical co- 

 lumns at the equator were to deposit the whole of their 

 watery stores, the moisture precipitated would cover 

 quantity of the surface of the earth only to the depth of 6.315 

 moisture inches. As we advance towards the poles, the quanti- 

 exuting t ty of moisture held in solution gradually decreases, in a 

 the . m * ratio which diminishes faster than the temperature, and 

 1'tmotphere w ' 1 ' c ' 1 se ' ms to have some relation to the mean quanti- 

 ' ty of rain in the different latitudes. The mean quanti- 

 ty of water existing in the vaporous state in a column 

 of air, having a square inch for its base, and reaching 

 to the summit of the atmosphere, may be reckoned by 

 the Table equal to 4 cubic inches ; and since the whole 

 surface of the globe is about 790,000,000,000,000,000 

 square inches, the whole moisture in the atmosphere may 

 be computed to be about 3,000,000,000,000,000,000 

 cubic inches, or 11,794 cubic miles of water. 

 Quantity of 96. Having thus formed a general estimate of the ab- 

 moisture solute quantity of moisture, existing at the same time 

 j n fa e atmospherical columns in the mean hygrometric 

 state of the air, we shall now endeavour to determine 

 the absolute humidity of the various strata of these co- 

 lumns, at different elevations, above the surface of the 

 earth. 



The observations which have been made on the hy- 

 grometric condition of the air, at considerable heights 

 above the level of the sea, though they evidently prove 

 that the quantity of moisture in the more elevated 

 strata of the atmosphere is extremely small, yet as they 

 generally have a reference to the state of the human bo- 

 dy, such as great thirst, the parched state of the skin, 

 and other effects of a similar kind, which might be pro- 

 duced either by absolute or relative degrees of dryness, 

 we must be satisfied, in the absence of the more precise 

 indications of the hygrometer, with the results deduced 

 from the general principles we have already establish- 

 ed, and still suppose that the same relative degree of 

 humidity pervades the whole atmosphere, as we have 

 endeavoured to shew, prevails near the surface of the 

 ground. The supposition, taken in conjunction with 

 that of an uniform decrease of temperature, amounting 

 to 3 3 .7 for every thousand feet of ascent, (the rate at 

 which the heat of the air diminishes according to the 

 best observations,) will enable us to compute the hu- 

 midity of the successive strata of the atmospherical co- 

 lumns, and explain the cause of the great dryness which 

 is observable in very elevated situations. 



97- That the mean relative humidity of the atmosphere 

 is nearly the same at various elevations, is neither im- 

 probable nor inconsistent with the few hygrometric ob- 

 servations of which we are in possession. Thus in a 

 range, which embraced about 7000 feet of difference of 

 level among the Alps, it appears, fromsome observations* 

 recorded by De Luc, that his hygrometer differed no 

 more than 10 at the upper and lower stations, though 

 the absolute quantity of moisture in the same volume of 

 air must have been thrice as -much in the one case as in 

 the other. The difference, which might have been occa- 

 sioned by partial changes in the hygrometric state of the 

 air, or even by the influence of temperature in modify- 

 ing the affinity of the whalebone for moisture, would only 

 have indicated, at the same temperature, a deviation of |th 

 from the same relative degree of humidity. Hence the 

 suppositions we have made, cannot lead to results differ- 

 ing greatly from the truth. The following Table, con- 

 structed on the principles we have laid down, and adapt- 

 ed to the mean temperature of the parallel of 45 of lati- " 

 : tilde, exhibits the mean quantity of moisture in grains 



contained in a cubic inch of airfor every 1000 feetofele- 

 vation, from the level of the sea to the height of 15,000 



fret. 



The first column contains the altitude in feet, and 

 the second the corresponding temperature ; the num- 

 bers in the third column express the elasticity of the 

 vapour, on the supposition that the point of deposition 

 for the various strata is 6 below the temperature, tlie 

 entire tension of vapour for the mean temperature of 

 each stratum being denoted by unity ; and the last co- 

 lumn shews the weight in grains of the moisture in a 

 cubic inch of air at the different heights. It appears 

 by the Table, that the absolute quantity of moisture 

 contained in a given volume of the air, at various heights, 

 is reduced about one half for every 4500 feet of ascent. 

 This great diminution of moisture is not detected im- 

 mediately by hygrometers, for reasons which we have ful- 

 ly explained in 89; but it is sensibly discovered by the 

 extreme thirst and parched state of the skin which it 

 occasions. Colonel Beaufoy describes the latter effects c olone ] 

 so well, in his account of his journey to the summit of Beaufoy's 

 Mont Blanc, that we shall make no apology for stating account of 

 in his own words the feelings he experienced : " We the extreme 

 had not proceeded far," says he, in giving an account dl 7 ncss he 

 of his ascent, " when the thirst, which, since our ar- or^hTsu'rn. 

 rival in the upper regions of the air, had been always m j t of 

 troublesome, now became intolerable. No sooner had Mont 

 I drank than the thirst returned, and in a few minutes Blanc, 

 my throat became perfectly dry. Again I had recourse 

 to water, and again my throat was parched. The air 

 itself was thirsty ; its extreme dryness had robbed my 

 body of its moisture. Though continually drinking, the 

 quantity of my urine was almost nothing; and of the lit- 

 tle there was, the colour was extremely deep. The guides 

 were equally affected. Wine they would not taste ; 

 but the moment my back was turned, their mouths 

 were eagerly applied to my cask of water." The cause Loss of 

 of these effects will be distinctly perceived, by attend- moisture bj 

 ing to the different circumstances in which a person is breathin S- 

 placed at the top and bottom of a lofty mountain. Near 

 the level of the sea, in the latitude of 45, we shall sup- 

 pose a person inhales air, which, in its mean hygro- 

 metric state, contains, according to the preceding Ta- 

 ble, .0026315 grains in a cubic inch, and expels it 



* Ideei ttir la Meteorolcgie, torn, ii. p. 9. 



