1054 TjRAySACTlOXS OF THE AifERtCAK INSTITUTE. 



"When we cover a tliermometer bulb with a thin piece of cloth, and 

 wet that cloth, we have an instrument which is sensible to the tem- 

 perature of the air in precisely tlie same degree that our lungs are ; 

 that is, a person may feel too. warm, or too cold, in a room the tem- 

 perature of which (as indicated by the common thermometer) is 

 seventy degrees ; for the same reason that, in such a room, the wet 

 bulb thermometer may indicate a temperature of seventy degrees, 

 or of only fifty-five degrees ; the first of which, seventy degrees, is too 

 Avarm, while the latter, fifty-five degress, is much too cold. 



The amount of difterence between the dry and wet bulb thermo- 

 meters depends directly upon the amount of moisture in the air. If 

 the air contains all the moisture that it is capable of holding, as often 

 occurs in wash and bathing rooms, it will be found that there is no 

 difference in the readings of the two thermometers ; but if the air is 

 very dry, as we often find it to be in artificially warmed aj^artmeuts, 

 the readings of the two thermometers may be very diflerent. 

 , 3d. The relative amount of moisture in the air. 



Air absorbs and holds in suspension watery vapor in the same 

 maimer that a sponge will hold water ; but the amount that a given 

 bulk of air will hold depends upon its temperature. Thus one cubic 

 foot of air, at thirty-two degrees, will hold in suspension but two 

 grains of water, while one cubic foot of air, at sixty-eight degrees, 

 will hold seven and one-half grains. When air, at thirty-two degrees, 

 has two grains of watery vapor in it, it is said to be saturated. When 

 air is saturated, that is, contains all the moisture that it is capable of 

 holding, we say that its relative humidity is 100 ; if it contains three- 

 fourths of the amount it is capable of holding, we say that its relative 

 humidity is seventy-five per cent ; or if one-half, fifty per cent, etc. 



"We see from this that the relative humidity of the air does not 

 express at all the absolute amount of watery vapor present. For 

 Instance, the relative humidity of air at zero may be, say, ninety per 

 cent, and yet contain less watery vapor than air at seventy degrees, 

 wdiose relative humidity is but thirty per cent. 



4th. The dew-point. 



A glass tumbler, filled with cold water in summer, is soon bedewed 

 with moisture, not, as is frequently imagined, because the water oozea 

 through the tumbler, but because the air around it is cooled, and its 

 moisture precipitated upon it. The same would occur in winter if 

 the tumbler were brought into a close room in which many ^^ersons 

 were assembled, and the air loaded with the accumulated vapor 



