488 SCIENTIFIC RECORD FOR 1883. 



" VII. The atmospheric pressure and its variations are cliniatologic 

 factors of secondary importance in strong contrast to the important part 

 that these elements play in meteorology. The mean annual pressure may 

 be given to within a millimeter as an index to the density of the air and 

 the facility of evaporation, but the variations that occur at any locality 

 are too small to have any direct sensible effect on animal and vegetable 

 life. It is only as a basis for the explanation of the distribution of other 

 climatic factors that we may need accurate barometric observations at 

 numerons stations. [The marked effect upon many persons of a gradual 

 removal of residence from lowlands to regions higher by several thousand 

 feet is daily exemplified by the experience of the numerous invalids who 

 resort to the Eocky Mountain plateaus and the higher portions of the 

 Appalachian Eange. The freedom from noxious dust floating in the 

 air and settling by its own weight or washed down by falling rain and 

 snow, is, in the absence of direct observations, approximately indicated 

 by the height of stations above sea-level or the annual barometric press- 

 ure, combined with the height above lowlands in the neighborhood; 

 these heights may of course be deduced from accurate barometric obser- 

 vations.] 



"VIII. The total effect of temperature, pressure, humidity, and wind 

 upon free water surfaces respectively in the shade and in the sun is to 

 produce an evaporation the measure of which should be of the highest 

 importance in climatology. The apparatus hitherto devised for measure- 

 ments of the amount of evaporation are, however, apparently very un- 

 satisfactory. [Some adopt instead the depression of the wet bulb as 

 indicative of the total effect of the preceding causes upon a moist sur- 

 face that is very similar to that of leaves — the human skin, &c. It is 

 perhaps proper to consider the depression of the dew-point temperature 

 as indicating the need the air has of moisture, but the elevation of the 

 wet bulb above dew-point, as showing the rate at which this need is 

 being supplied, and the elevation of the air temperature above the wet 

 bulb, as showing the rate of strain that every surface is under in its 

 effort to give up its moisture to the absorbing atmosphere.] 



"IX. (1) The constitution of the air, so far as its dry gases are con- 

 cerned, is too uniform throughout the world to allow of its entering as 

 a factor in studying climatic differences ; it varies but a fraction of 1 

 per cent, from 21 volumes of oxygen and 79 volumes of nitrogen, with 

 0.03 of one volume of carbonic-acid gas. The most important variable 

 in the air is the amount of aqueous vapor (see V (1), above). The per- 

 centage of volume of vapor to dry gases is given by dividing the tension 

 of vapor by the barometric pressure. The result is, in extremely moist 

 climates, equivalent to a dilution of the air to an extent of perhaps 3 

 per cent, of its volume. Thus for Batavia in one volume of the dry gases, 

 we should have oxygen 21.0, nitrogen 79.0 per cent., but in one volume 

 of the whole atmosphere, oxygen 20.4, nitrogen 76.8, aqueous vapor, 2.8 

 per cent. The direct effect of the slight change in oxygen must be im- 

 perceptible. 



