ing the first bulb and the tube tilled with the vapor of ether. A little ether is then dropped from the bottle on the muslin-covered bull); eva porn t ion instant ly takes place, and the temperature is reduced, thus condensing the vapor inside; a fresh evaporation from the other bulb till; the vac uum. and the vapor is again condensed by drop ping more ether on the muslin covering. the process being repeated until the temperature of the hull) containing the thermometer is so re duced by successive evaporations (see EVAPORt TION) that deer begins to be formed on the out side of the bulb. At the instant this occurs, the height of the mercury in the two thermometers is accurately noted. the one giving the dew-point and the other the temperature of the air. There are other forms of dew-point apparatus. all depending upon this same prin ciple. mid the hygrometer of Reguatilt was de vised to. remedy some of the defects of the Dan iell apparatus. In this instrument a current of air is used to increase the evaporation of the ether. and instead of a glass tube and bulb there is a brightly polished thimble of silver at the base of one of the thermometers, which contains the ether. The air bubbles through the liquid. and the rate of evaporation becomes much more rapid. Knowing the dew-point, a reference to the table will furnish the pressure of aqueous vapor at that temperature, and other tables or the appli cation of a formula will give the amount of mois ture in a givot quantity of air.
Instead of the dew-point apparatus, the wet and dry bulb thermometers, or psychrometer, is now generally employed in making ordinary observations, especially by meteorologists. The instrument consists of two ordinary eters—one has its bulb bare, and thus shows the temperature of the air, while the other has its bulb covered with muslin which is kept wet with water. The thermometers are often so united that they can be rapidly revolved in order that the wet bulb may be exposed to a stant current of air. In the illustration is shown the sling psychrometer issued by the United States Weather Bureau. where the thermometers are mounted in such a manner that they can be whirled about the hand of the observer. The
oration from the muslin and the consequent cooling of the bulb being proportional to the ness of the air, the difference tween the readings of the two thermometers is greatest when the air is driest, and zero when it is completely saturated. The readings of the psychrometer and the dew-point apparatus at the same time have been fully compared. and it has been ascertained that the vapor pressure at the dew-point of air is equal to the vapor pressure corresponding to the ture of the wet-bulb ter minus the number MI I. mul tiplied by the difference in degrees between the dry and wet bulb thermometers. This rule is stated as applying when the psychrometer is whirled.
and when the pressure of the barometer is equal to 30 inches. From the difference in readings between the thermometers the vapor pressure can be calculated by formulas which. with certain modifications, are based on those used by Re gnault.
Ferrel's formula, used by the 'United States Weather Bureau in calculating its tables, is as follows: Let p =vapor pressure of saturation at dew-point tem. perature.
vapor pressure of saturation at wet-bulb temper. ature.
P =barometric pressure, taken as 23.4 in computing the tables.
t = readier of dry thermometer.
r= reading of wet thermometer.
Then for dew points below 32' p = p'—.000360 (t—t') (1+.000G3 t') and for dew points above 32° p=p'—.000367 P (t—r) In practiee, however, it is usual merely to take the difference between the wet and dry bulb thermometers, and refer at once to the tables, which will give the relative humidity, the amount of moisture in a given quantity of air, and other defired data. These tables and full directions for practical hygrometry are contained in the publications of the United States Weather Bu reau, to which the reader is referred for further information, while nearly all the larger treatises on physics contain satisfactory treatment. of the subject. Consult also Preston, Theory of Heat (New York. 169-1). See II l'311 MTV.