Atmospherical Phenomena, 255 



pressure of the atmosphere, the wind, and other circumstances ex- 

 ercise, it is true,aninfluenceon therapidity of the evaporation, but 

 are without effect as to the amount that a space in a state of satu- 

 ration can take up ata given temperature. This state of saturation 

 is however but seldom to be found in the atmosphere, and for the 

 most part the amount of vapour lies between it and perfect 

 dryness. When we wish to ascertain the amount of moisture 

 in the air, we must necessarily distinguish two separate quanti- 

 ties. We must first of all determine the amount of moisture 

 which the air really contains, whether we deduce the weight of 

 a cubic foot of watery vapour by simultaneous observation of 

 the thermometer and hygrometer, or whether, what is always 

 more suitable, we mark the pressure of the whole atmosphere 

 by the length of the quicksilver column, in the same way 

 as this is done for the pressure of the air by the barometer. 

 The quantity thus found, gives the amount of vapour or the 

 absolute humidity. Although this quantity forms the founda- 

 tion of the investigation, yet it is by no means sufficient. For 

 as the quantity of vapour, which a cubic foot of air contains 

 when in a state of saturation, is greater at a higher tempera- 

 ture than when the thermometer stands at a lower point, so the 

 same quantity of vapour, which, at a low temperature of the 

 air, characterizes a very moist atmosphere, can, during the 

 summer, belong to a very dry atmosphere. Hence, in order to 

 determine this important element, we seek for the amount of 

 vapour which the air, when saturated, can contain at the mo- 

 ment of observation, and by it divide the absolute amount of 

 vapour; the quotient gives by per-centage the relative humidity 

 or the relative amount of vapour in the air. 



As in this manner we obtain the relations of humidity in the 

 atmosphere under a double point of view, it becomes possible 

 for us to point out some laws with greater distinctness than was 

 done by meteorologists in earlier times. When, in the morn- 

 ing, the sun acts on the surface of the earth, the surfaces of 

 water thus heated evaporate, and the amount of vapour in the 

 lower strata of air increases rapidly ; but at the same time the 

 air becomes heated, and as the pressure of the air off^ers some 

 opposition to the evaporation, the heat increases much more 

 quickly than the amount of vapour : the air, therefore, not- 

 withstanding the accession of moisture, becomes relatively 



