HYGINtJS. 392 HYGROMETER. consisting of 277 mythological legends, valuable cliiflly l^-ausc of llie use- nmilu of the firifk tra-iodics ; the other, an astronomical treatise cnlilli-J I'octicon Astronomicon Lihri IV., of little value now. The FabiiUr are edited by Schmidt (Jen.T, 1872), and the .-Is^roiioniioa b_v Bunte (Leipzig, 1875). Another Ilyginus, known as- fJroiiKilicus. lived probably in the second cen- ttirv, and wrote a work on surveying; edited by Gciiioll (Leipzig, 1879). HYGIinjS, Saint. Pope, or Bishop of Rome, abiiut the middle of the second century, possibly 137141. 'ory little is known about him. HYGROM'ETER (from Gk. iyp6(, hygros, moist + fitTfrnv, mtlron, measure). An instru- ment for measuring the quantity of moisture in the atmosphere. In addition to the gases of which it is composed, the air contains 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 chiclly on the temperature, being at a minimum on cold days, and on warm days form- ing as much as one-fiftieth part of the air. This vapor has a certain tension or pressure depend- ing upon the teni|)erature, and when the latter is sullieiently reduced condensation takes place and dew 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 lx;low 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 ascer- tained experimentally, we can ti.se it to determine the pressure of the vapor at that temperature, as the pressure of water-vapor and its relation to tempeiature has formed the subject of extensive and elaborate experimental researches 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 vapr)r pressure, consequently a ratio between the vapor pressures at an observed temperature, and at the temperature of condensation for the atmnspheric condition at the time of observation, would give the relative humidity. It is in terms of relative humidity that the amount of moistiire 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. Hygrometers can be divided into several classes. There are those in which use is made of the prof)erty 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 physici.st, and a somewhat similar instrument was devised by IkHuc in which a strip of whalebone was em- ployed. The instrument.s are known as hygro- scojies, and may be u-sed indoors or where only an approximate reading is desired. Another cl.i.ss of hygrometers is based on the principle 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 consi.sts of a series of drj-ing-lubes containing some hygroscopic mate- rial, as substances which readily absorb mois- ture are termed, through which air 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, and potassium or so- dium oxide. The amount of moisture is deter- !nined by the increas*' in weight. The class most generally used, however, includes ap|iaratus where the temperature is reduced until con- densation takes place, or this temperature can be determined from the temperature of evapora- tion. Knowing the dew-point for a given condi- tion of the atmosphere, the other data can be readily ascertained. The Daniell hygrometer, which figures exten- fivcly in the earlier works on physics and meteor- ologi", consists of two bulbs connected by a bent tube, as represented in the figure,, and inclosing a thermometer together with Bome ether, and vapor of ether, the air having been ex- pelled before the tube was sealed. One bulb is either black- ened or coated with metal, wliile 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 opposite bulb, leav- ing the first bulh and the tube filled with the vapor of ether. A little ether is then dropped from the bottle on the muslin-covered bulb; evaporation instant- ly takes place, and the temperature is reduced, thus condensing the vapor inside; a fresh evaporation from the other bulb fills the vac- uum, and the vapor is again condensed by drop- ping more ether on the muslin covering, the process being repeated until the temperature of the bulb containing the thermometer is so re- duced by successive evaporations (see Evapob.v- tion) tjiat dew begins to be formed on the out- side of the bulb. At the instant this occurs, the height of the mercury in the two thermometers is accurately noted, the one giving the dew-point temperature, and the other the temperature of the air. There are other forms of dew-point apparatus, all depending upon this same prin- ciple, and the hygrometer of Regnault was de- vised to remedy some of the defects of the Dan- iell apparatus. In this instrument a current of air is used to increase the evaporation of the ether, and instead of a glass tube and bulb there is a brightly polished thimble of silver at the base of one of the thermometers, which contains the ether. The air bubbles through the liquid, and the rate of evaporation becomes much more rapid. Knowing the dew-point, a reference to the table will furnish the pressure of aqueous vapor at that temperature, and other tables or the appli- DA.VIELL HTOROUETEB.
