Hygrometers. These are instruments to determine the amount of moisture in the air. One much used is called the wet bulb hygrometer, and consists of two similar delicate mercurial thermometers, the hull) of one of which is covered with muslin, and is kept constantly wet by water, led on to it by a ring from a tube in the Centre. The evaporation of the water from the wet bulb reduces the temperature of that thermometer to which it is attached in proportion to the dryness of the air, and consequent rapidity of evaporation. The other thermometer indicates the actual temperature, and the difference being noted, a mathematical formula enables us to determine the dew point. But the most | complete and beautiful instrument for this use is that of Mr. Daniell, which is here represented. The long limb ends in a bulb which is made of black glass, th t the condensed vapour may be more easily seen on it. It contains a portion of ether, into which dips the ball of a small and delicate thermometer contained in the cavity of the tube. The whole instrument contains only the vapour of ether, air having been removed. The short limb carries an empty bulb, which is covered with muslin. On the support is another thermometer, by which we can observe the temperature of the air. When an observation is to be made by this instrument, a little ether is poured on the muslin: this evaporates rapidly, and the bulb becomes cooled. After a time, through the cooling agency, dew begins to deposit on the black glass, and the point at which this takes place is determined by the included thermometer.

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