PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 623 



two grains of watery vapor in it, it is said to be saturated; if it 

 has but half this amount, we call it half saturated. When air is 

 saturated, that is, contains all the moisture that it is capal)le of 

 holding, wo 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 75 per cent; or if one-half, 50 per cent, 

 &c., &c. 



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 90 per 

 cent, and yet contain less watery vapor than air at 70 degrees, 

 whose relative humidity is but 30 per cent. 



4th. The dew-point. 



"A glass tumbler filled with cold v/ater, in summer, is soon 

 bedewed with moisture, not, as is frequently imagined, because the 

 water oozes 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 persons were assembled, and the air loaded with the 

 accumulated vapor exhaled from their lungs and skin. From the 

 same cause, the cold windows of a crowded lecture-room are con- 

 stantly covered with minute drops of water, which soon collect 

 to2;ether and run down the o-lass in streams. 



The highest point of the thermometer at which vapor begins to 

 be deposited by the air, is called the deiv-point ; it is the point at 

 which dew begins to form." 



5th. The absolute amount expressed in grains of water contained 

 in a cubic foot of air, at the temperature, and relative humidity, as 

 shown by the instrument. 



6th. The force of vapor expressed in inches of water; in other 

 words, the depth of water that would be deposited upon the earth 

 by the condensation of all the vapor held in the atmosphere. 



Directions for Maintaining a Healthy Atmosphere. 



A full-grown person, in health, should have a supply of fresh 

 air amounting to at least one thousand cubic feet per hour; in 

 sickness much more is required. 



To supply this amount of air to an apartment of the average 

 size of our sitting-rooms, and under ordinary conditions, there 

 should be an inlet and an outlet, each of an area of at least one 

 square foot; both orifices should be so arranged as to bo under 

 perfect control, by means of blinds, registers, or valves. 



