HYGROMETER (from Ck. i;p6c, hygros, moist pirpor, m, (run. ineas.urel. An instil merit for measuring the quantity of moisture in the atmosphere. In addition to the gases of which it is composed. the air eontains the vapor of water, which is invisible except when it is con densed in the form of fog or cloud. The amount of this aqueous vapor varies greatly, and de pends chiefly on the temperature, being at a minimum on cold days, and on warm (lay, form ing as much as onctiftieth part. of the air. This vapor has a certain tension or pressure depend ing upon the temperature. and when the latter is sufficiently reduced condensation takes place and is formed. The temperature at which the air contains a maximum of moisture for a given pressure is called the temperature of saturation for that pressure. and if the temperature is re duced below this point condensation takes place. The temperature at which condensation occurs is known as the dew-point (see DEW) tempera ture. The less moisture the air contains the lower will be the dew-point. and when this is aseer tained experimentally, we can use it to determine the pressure of the vapor at that temperature, as the pressure of water-vapor and its relation to temperature has formed the subject. of extensive and elaborate experimental researehes that are now embodied in carefully prepared tables. It is known that the quantity of water contained in the air varies directly (or nearly) as the vapor pressure. consequently a ratio between the vapor pressure; at an observed tA•perature, and at. the temperature of eondensation for the atmospheric condition at the time of observation. would give the relative humidity. it is in terms of relative humidity that the amount of moisture in the atmosphere is expressed in the daily weather reports, and the chief function of the hygrometer is to afford such data as will enable us to determine this quantity.
Ilygrometers can be divided into several classes. There are those in which use is made of tl e property of certain substances of altering their dimensions or weight upon absorbing mois ture from the air. Of this class the most used
are those in which a hair expands and contracts according as the air is more or less moist, and moves a pointer or index over a scale. and indi cates relative humidity direct. This form was invented by Saussure, the Swiss physicist, and a somewhat similar instrument was devised by Delue in which a strip of whalebone was em ployed. The instruments are known as hygro scopes, and may he used indoors or where only an approximate reading is desired. Another class of hygrometers is based on the prineiple that certain chemicals will absorb moisture. and the amount thus abstracted from a given volume of air will give the quantity present in the at mosphere. The apparatus consists of a series of drying tubes containing mime hygroscopic mate as substances which readily absorb mois ture are termed, through which nir is forced by means of an aspirator or other device. The sub stances which may lie used for this purpose are strong sulphuric acid in pumice-stone, calcium chloride, phosphoric acid, nnd potassium or so dium oxide. 'fbe amount of moisture is deter mined by the increase in weight. The class most generally used, however, includes apparatus where the temperature is reduced until con densation takes place, or this temperature can be determined from the temperature of evapora tion. linowing the dew-point for a given condi tion of the atmosphere, the other data can be leadily ascertained.
The Daniell hygrometer. which figures exten sively in the earlier works on physics and meteor (dog.% consists of two bulbs connected by n bent tube. as represented in the figure, and inelosing I ogether with some ether. and of the air having been ex pelled before the tube was sealed. One bulb is either black ened or coated with metal, while the other is covered with muslin. The ob server's hand is placed for a short time on the muslin covered bulb to drive the ether into the ocite . .