. Johnson's new universal cyclopaedia : a scientific and popular treasury of useful knowledge. ugh observations withthis instrument give the clastic force of vapor of the airwith great precision, yet it is not as convenient to use ascould be desired for thedaily registration of theliygrometric conditionof tho air by ordinaryobservers. To obviatethis difficulty the wetand dry bulb hygrom-eter has been was devised by Ma-son, and consists of twothermometers, of thesame size of bulb andbore, placed alongside ofeach itther (Kig. -J)* nohaving a naked drybulb, and the other abulb covered
. Johnson's new universal cyclopaedia : a scientific and popular treasury of useful knowledge. ugh observations withthis instrument give the clastic force of vapor of the airwith great precision, yet it is not as convenient to use ascould be desired for thedaily registration of theliygrometric conditionof tho air by ordinaryobservers. To obviatethis difficulty the wetand dry bulb hygrom-eter has been was devised by Ma-son, and consists of twothermometers, of thesame size of bulb andbore, placed alongside ofeach itther (Kig. -J)* nohaving a naked drybulb, and the other abulb covered with finemuslin, moistened at thetime of observation, orkept continually moistby the ascent of waterin a cotton wick, asshown in the to the evapora-tion from the coveredbulb, its temperature willbo generally below thatof the naked bulb, andthis difTcrenco will begreater when tho air i>very dry. When it \>wet, or near the dew-point, the evaporationwill be very slow, andthe two thermometerswill indicate nearly thosame temperature. It has been objected to this instru- Masons wet and dry bulb hygrom-eter. ment that the evaporation is greater when there is a cur-rent of air, but it must be remembered that the same cur-rent tends to elevate the temperature of the covered bulb innearly the same proportion. By compiiring the indicationsof this instrument with that of the Uegnault hygrometer, atable may be formed experimentally by whieh the dew-point and elastic tension of the air can be determined atonce by inspection. l>r. Apjohn of Trinity College, Dub-lin, who has given much attention to this subject, has ob- d htained the following formulas: f =f—^ ^.»7 ^ which/ denotes the maximum elasticity or tension of vapor corre-sponding to the temperature of the wet-bulb thermometer;r the elasticity of vapor present in the air which we de-sire to find ; d denotes the difference in degrees of Fahren-heits scale between the two thermometers ; and /* the heightof
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