68 NEGRETTI AND ZAMBKA, HOLBOJiN VIADUCT, E.G., 



INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE ATMOSPHERE. 



THE instruments used for observing the amount of moisture contained in 

 the atmosphere are called Hygrometers. They are without doubt of all 

 Meteorological instruments the most useful and valuable. 



To ascertain with exactness the Hygrometric condition of the air is of the 

 utmost importance both to the Physician and Agriculturist. By observing the 

 varying amount of vapour or moisture in the air, the one is enabled to regulate 

 its condition as best suited to his patient's requirements, and the other by closely 

 watching the movements of the Barometer in connection with the Hygrometer 

 can anticipate probable atmospheric changes that may prove beneficial or 

 injurious to his crops. 



There are many Hygrometers constructed as Weather Indicators only, 

 simply showing the approximate condition of the air if it be wet or dry. Such 

 instruments, however ingenious, are not of any scientific value. For more exact 

 and precise observation the Hygrometers of Daniell, Regnault, and Mason are 

 chiefly used the latter, viz., Mason's, from its extreme simplicity is now 

 universally in use, and in connection with the valuable tables compiled by 

 James Grlaisher, Esq., F.B.S., the dew point can be ascertained with great 

 exactness and ease. 



In connection with the Hygrometer, the dew point will be frequently 

 spoken of. This may be described in a few words as the amount of water 

 which the air can sustain in an invisible form increasing with the temperature ; 

 but for every definite temperature there is a limit to the amount of vapour 

 which can be thus diffused. When the air is cooled the vapour present may be 

 more than it can sustain ; part will then be condensed either in the form of 

 dew, rain, hail, or snow. The temperature which the air has when it is so fully 

 saturated with vapour that any excess will be deposited as dew, is called the 

 dew-point. 



" To measure the quantity of dew deposited each night, an instrument is 

 used called a Drosometer. The most simple process consists in exposing to the 

 open air bodies whose exact weight is known, and then weighing them carefully 

 when covered with dew. According to Wells, locks of wool, weighing about 

 eight grains, are preferred, divided into spherical masses of the diameter of 

 about two inches.'' Kcemtz. 



