. The principles of hygiene; a practical manual for students, physicians, and health-officers . base is greater; (2) when theair is dry, for then it is denser than when it is moist;(3) when in any way an upper current sets in toward agiven area, for this compresses the strata underneath. Conversely, the barometer is low (1) when the lowerstrata are heated, causing the surfaces of equal pressureto rise, and the upper layers to slide off, for by this meansthe mass of air pressing on the area below is reduced;(2) when the air is damp, for as the density of aqueousvapor at o° C. temperature and 76
. The principles of hygiene; a practical manual for students, physicians, and health-officers . base is greater; (2) when theair is dry, for then it is denser than when it is moist;(3) when in any way an upper current sets in toward agiven area, for this compresses the strata underneath. Conversely, the barometer is low (1) when the lowerstrata are heated, causing the surfaces of equal pressureto rise, and the upper layers to slide off, for by this meansthe mass of air pressing on the area below is reduced;(2) when the air is damp, for as the density of aqueousvapor at o° C. temperature and 760 millimeters pressureis , air being 1, the mixture is lighter the morevapor it contains, and consequently damp air does not HUMIDITY OF THE A TMOSPHERE. 37 press as heavily as dry air on the unit of area below; (3)when the air, from any cause, has an upward movement,for this, of course, acts in the same manner as when thelower strata are heated. Humidity of the Atmosphere; Hygrometry.—Hygrometry is the determination of the amount ofaqueous vapor in the air. Hygrometers are either. FlG. 2.—Daniells hygrometer : a, bulb surrounded with cotton cloth; b, ther-mometer; c, bulb containing thermometer. direct, as Daniells (Fig. 2), Dines, and Regnaults,and determine directly the dew-point of the atmosphere;or indirect, as the wet- and dry-bulb thermometer, orpsychrometer (Fig. 3), and the hair hygrometer (Fig. 4).The important items of information to be derived fromobservations with hygrometers are: The dew-point, thevapor-tension or absolute humidity, and the relative hu- 38 AIR. midity of the atmosphere. The dew-point is that tem-perature at which the air is saturated with moisture, sothat the least further fall in temperature causes a deposit of moisture in the form ofdew. The higher the tem-perature of the air, the largerthe amount of water it cancontain in the form of vapor;and if the temperature be low-ered, the amount of moistureremaining the same, eventu-ally a poi
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Keywords: ., bookcentury1900, bookdecade1900, booksubjecthygiene, bookyear1901
