HYGROMETER. 393 HYKSOS. cation of a formula will give the amount of mois- ture in a given quantity of air. Instead of the dew-point apparatus, the wet and drj- bulb thermometers, or psychrometer, is now generally employed in making ordinary observations, especially by meteorologists. The instrument consists of two ordinary thermom- eters — one has its" bulb bare, and thus shows the temperature of the air, while the other has its bulb covered with muslin which is kept wet with water. The thermometers are often so united that they can be rapidly revolved in order that the wet bulb may be exposed to a con- stant current of air. In the illustration is shown the sling psychrometer issued by the United States Weather Bureau, where the thermometers are mounted in such a manner that they can be whirled about the hand of the observer. The evap- oration from the muslin and the consequent cooling of the bulb being proportional to the dry- ness of the air, the difference be- tween the readings of the two thermometers is greatest when, the air is driest, and zero when it is completely saturated. The readings of the psychrometer and the dew-point apparatus at the same time have been care- fully compared, and it has been ascertained that the vapor pressure at the dew-point of air is equal to the vapor pressure WEATHER corresponding to the tcmpera- ture of the wet -bulb thermome- ter minus the number .011. mul- tiplied by the difference in degrees between the dry and wet bulb thermometers. This rule is stated as applying when the psychrometer is whirled, and when the pressure of the barometer is equal to 30 inches. From the difference in readings between the thermometers the vapor pressure can be calculated by formulas which, with certain modifications, are based on those used by Re- gnault. Ferrel's formula, used by the United States Weather Bureau in calculating its tables, is as follows : Letp=vapor pressure of saturation at dew-point tem- perature. p'=^ vapor pressure of saturation at wet-bulb temper- ature. P ^barometric pressure, talien as 29.4 in computing the tables. t ^ readinpT of cir.v thermometer, t'^ reading of wet thermometer. Then for dew points below 32^ p —p"— .000.160 P (<—«') ( 1-)- .00065 «') and for dew points above 32° p=p'-.0OO367 V{t~n (H-J^") In practice, however, it is usual merely to take the ditlerence between the wet and dry bulb thermometers, and refer at once to the tables, which will give the relative humidity, the amount of moisture in a given quantity of air, and other d^ired data. These tables and full directions for practical hygrometry are contained in the BTKEAr SLING PSY CHHOMETEB, publications of the United States Weather Bu- reau, to which the reader is referred for further information, while nearly all the larger treatises on physics contain satisfactory treatment of the subject. Consult also Preston, Theory of Beat (Xew Vork, 1894). See Humidity. . HY'GROPHYTES (from Gk, i;p6c. hygros, moist -f yiror, phyton, plant). Plants which grow naturally in regions where the atmosphere is moist. Hygrophytes differ from hydrophytes in that they are not necessarily associated with water in the liquid state. Ilygrophytic leaves show a number of peculiarities" in contra.st with xeropliytic leaves. For example, hygrophytie leaves do not suffer, but rather thrive, in very moist atmospheric conditions. The leaves them- selves are also easily, moistened. This type of leaf is very characteristic of the tropical" rainy forests, Xerophytic leaves, in contrast with hy- grophrtic leaves, are not readily moistened, and commonly die if subjected to a moist atmosphere. See Hydrophyte. HY'GROSCOPE. See Htgrometee. HYKSOS, hik'soz. The name of a dynasty of Egyptian Kings, who are known also as the Shepherd Kings. JIanetho, of whose annals an excerpt is preserved in Josephus, narrates that foreign conquerors from Asia took possession of Egypt who were called Hykussos. This name seems to mean 'foreign kings' (hyku-khesou) ; the explanation, based on the mutilated form Hyksos and interpreting sos as Egj'ptian shos, 'shepherd.' although attributed to Manetho, seems to have originated at a later time. Con- sequently, we have in this no basis for deter- mining the nationality of those foreigners, 'of iinkno«-n race,' as ilanetho says. Usually, his- torians, following this later etymology and the insertion in Manetho ("they are said to have been Phoenicians or Arabs"), consider them as nomadic Semites, either Arabs or Canaanites, recently settled in Palestine, Others have sup- posed them to be Turanians, thinking that they found Mongolian features on statues which were for a long time attributed to the Hyk(us)sos, but are now generally recognized as belonging to an earlier period. Xothing positive can be said as to the origin of this people: it is only known that the conquerors had first founded an empire in Syria (coming from the north from Asia Minor?) before conquering Egj-pt. This explains why many Semites came thither along with them and, later, under their rule. The date of their conquest is doubtful. From the confused ex- tracts from Manetho, they yould seem to have formed Dynasties Fifteen and Sixteen, lasting 2,5!) and 251 years, respectively, and would thus have begim about 2100 B.C. But Dynasty Six- teen seems to be an erroneous repetition, so that only Dynasty Fifteen with six (?) kings re- mains, and for these 250 years arc certainly too much. Taking into consideration the recently discovered Sothiac date for DjTiasty Twelve, not nuich more than a century would remain for them, and therefore the invasion of the foreign- ers ought to be placed at about 1700 n.c. The reports of the cruelties and devastations, which, according to Eg-ptian tradition, accompanied the conquest, are untrustworthy. In one point, however, the traditional account has been con- firmed : the invaders built a strong fortress on the northeastern frontier of the delta, in the
