for the weight of a cubic foot of vapor at temperature 60°, and under a pres sure of 0.56 of an inch of the mercurial column.
The following table, abridged from Daniell's Meteorological Essays, shows the force of tension, weight, and expallifion of aqueous vapor, at different tempera tures, on Fahrenheit's scale.
Having thus explained the principle of the common hygrometer, we will now de scribe one or two of the forms under which it has been most frequently con structed. Daniell's hygrometer consists of two thin glass balls of I+ inch diameter, connected together by a tube having a bore about ith of an inch. The tube is bent at right angles over the two balls, and the arm contains a small thermometer whose bulb, which should be of a length ened form, descends into the ball. This ball, been about two thirds filled with ether, is heated over a lamp till the fluid boils, and the vapor issues from the capillary tube which terminates the ball. The vapor having expelled the air from both balls, the capillary tube is hermeti cally dosed by the flame of a lamp. The other ball is to be covered with a piece of muslin. The stand is of brass, and the transverse socket is made to hold the glass tube in the manner of a spring, al lowing it to turn and be taken out with little difficulty. A small thermometer is inserted into the pillar of the stand. The manner of using the instrument is this : after having driven out all the ether into the ball by the heat of the hand, it is to be placed at an cpen window, or out of doors, with the ball so situated that the surface of the liquid may be on a level with the eye of the observer. A little ether is then to be dropped on the cov ered ball. Evaporation immediately takes place, which producing cold upon the covered ball causes a rapid continuous condensation of the ethereal vapor in the interior of the instrument. The con sequent evaporation from the included ether produces a depression of tempera ture in the ball, the degree of which is measured by the thermometer. This ac tion is almost instantaneous, and the thermometer begins to fall in two seconds after the ether has been dropped. A de pression of 80 to 40 degrees is easily pro duced, and the ether is sometimes ob served to boil and the thermometer to be driven below zero of Fahrenheit's scale. The artificial cold thus produced causes a condensation of the atmospheric vapor up on the uncovered ball, which first snakes, its appearance in a thin ring of dew co incident with the surface of the ether. The degree at which this takes place must be carefully noted. In very damp
or windy weather the ether should be very slowly dropped upon the ball, oth erwise the descent of the thermometer will be so rapid as to render it extremely difficult to be certain of the degree. In dry weather, on the contrary, the ball re quires to be well wetted more than once, to produce the requisite degree of cold.
The instrument which has now been described is extremely beautiful in prin ciple ; but it may be doubted whether, even when the greatest caution is ob served, the temperature which it indi cates is precisely that at which the depo sition of dew takes place. The deposi tion first occurs in a narrow ring on a level with the surface of the ether in the ball b, thereby indicating that the elms is colder at the surface than a little under it. But if the temperature is not uniform throughout the ball, it is evident that only a small part of the bulb of the ther mometer can be placed in the point when the greatest cold exists ; consequently the temperature indicated by the ther mometer will be greater than is necessary for producing the deposition of moisture • in other words the dew point will be given too high.
Various attempts have been made tc obviate the defects of Daniell's hygrome ter, but hitherto without much success. The apparatus proposed by Pouillet may be described as follows : A small cup C C, formed of gold, and extremely thin, is fixed to a little collar of ivory 11 B, ported on a stand. The stem of an verted thermometer T T scends through a perforation in the bottoih of the cup, and is fitted closely into it and sealed, the ball of the thermometer being placed at the centre of the cup. In order to prevent the mercury from separating, a small portion of the air is left in the stem. When an servation is to be made, phuric ether is poured into the cup ; and in consequence of the rapid evaporation which takes place, a considerable de of cold is produced, and deposition takes place on the outside of the cup. The degree of the thermometer at the instant the bright ness of the metal begins to be dimmed gives the dew point. The correctness of the indication depends on the identity of temperature of the ether, the metal of the cup, and the thermometer. Bright is found to answer the purpose bet ter than any other metal.
As the hygrometer is one of the prin cipal instruments in meteorological re searches, its theory and the best form of its construction have been the subject of frequent discussion in the various scien tific journals.