the hair is fixed, and to the other is suspended a small weight. A grooved wheel or pulley carrying an index is placed so as to be moved by the hair as it contracts or expands. Various other hygrometers of this class have been devised, some on the principle of determining the moisture by the increased weight imparted to bodies by its absorption, and others by the torsion thereby induced in cords and in vegetable fibres; but all these methods have proved very imperfect.-Two other methods are to be noticed by which the humidity of the air is ascertained. The first depends on the determination of the dew point, or the degree of temperature to which the air must be reduced that its moisture shall begin to separate and condense upon cold surfaces. This difference alone is sometimes used to express the dryness of the air, as affording an indication of how near it is to its point of saturation. In temperate regions this sometimes amounts to 30°; but in a dry and hot climate, under the lee of cold mountains which first strip the air of its moisture, it amounts to 60° or more; such is the case upon the hot plains of the Deccan, to which the air is brought from the other side of the Ghauts. Cooled down upon these to a low temperature, its moisture is precipitated in rain and snow, and when immediately after this it is raised to a temperature of 90°, it is found that no deposition of moisture again takes place until the temperature is reduced to 29°. The observation, however, is used to furnish more exact results. Tables have been prepared with the utmost care which give the elastic force of aqueous vapor at different degrees and even tenths of degrees of temperature, expressed in the height of a column of mercury the vapor sustains. The temperature of the dew point of the air being ascertained, the elastic force corresponding to this temperature in the table represents the absolute humidity of the air, and may be converted into the actual weight of moisture to the cubic foot under a given barometric pressure by the formula prepared for this purpose, or directly by the tables constructed to reduce the labor of the calculation. By comparing the elastic force obtained from the table with that corresponding to the temperature of the air itself, the ratio between the two expresses the relative humidity of the air. This also is ascertained at sight by the tables specially constructed for this object. The most highly approved hygrometrical tables are those derived from the experiments of V. Regnault, made by direction of the French government to determine the expansive force of steam at different temperatures, which is also that of the vapor suspended in the air at the same temperatures. These tables are published in Regnault's Études sur l'hygrométrie, in the Annales de chimie et de physique (1845); and formula also are given from which other tables, beside that of the elastic forces, have been prepared by others. The most complete series of these is furnished in the volume of "Tables, Meteorological and Physical, prepared for

These

the Smithsonian Institution by Arnold Guyot, P.D., LL.D.," and published in the "Smithsonian Miscellaneous Collections," 1858. In the same series is also presented the table of elastic forces of vapor deduced from the experiments of Dalton, together with others based upon it, and in general use in England. are also found in Glaisher's "Hygrometrical Tables" (London, 1847), and in the Greenwich volumes. Various forms of the dew point instrument or hygrometer have been devised. That of Prof. Daniell, which has been much used, is of the following construction. A bent tube, blown out at each end to a bulb, is laid across the top of a pillar, which serves as a stand, the two bulbs hanging down one on each side. One tube is long enough to contain a delicate thermometer, the bulb of which terminates in some ether contained in the external bulbs. By boiling the ether before closing the tube the air is nearly expelled. When in use the empty bulb is covered with a piece of muslin, which is kept wet with ether. The evaporation of this condenses the vapor within, causing the liquid in the other bulb to evaporate and grow cool. The bulb becomes at last suffieiently cool for the moisture to condense upon it, and the instant this makes its appearance in the form of a ring of dew encircling the bulb at the level of the surface of the ether, the temperature is to be noted by the thermometer within, while that of the air is observed upon another thermometer attached to the stand. Another observation of the enclosed thermometer is made as the dew disappears by the bulb returning to its former temperature; and the mean of the two observations will give a close approximation to the dew point.-A better instrument is that of Regnault. Two glass tubes are suspended by a small tubular arm near the top of each, both opening into the hollow stand that supports the tubes. A pipe for exhausting the air by means of a sort of bellows or the flow of water connects with the hollow in the stand by an opening near its base. The two tubes are closed, each with a cork through which a thermometer tube is fitted, the bulb in one reaching nearly to the bottom. Over the lower end of this one a very thin and highly polished thimble of silver nearly 2 inches long is fitted, and a fine tube open at each end is passed through the cork, reaching from the external air nearly to the bottom of the tube. Ether is poured into this bulb, covering the lower end of the thermometer, and rising an inch or two higher than the upper edge of the silver thimble. To determine the dew point, the apparatus for .exhausting the air from the hollow stand is set in action. This causes the air to pass through the fine tube, and bubble through the ether, keeping it in motion and taking up its vapor. The liquid, the thermometer bulb, and the silver coating of the tube equally feel the reduced temperature, and the instant this reaches the dew point, the whole surface of the silver is covered with moisture.

The temperature of the thermometer placed in the ether is then observed, while the other marks the temperature of the air. By stopping the current of air the temperature rises, and the moisture disappears from the silver. The thermometer is to be noted again, and the mean of the two observations taken for the dew point; or several trials may be made in rapid succession. To avoid affecting the result by the warmth radiated from the body, a small telescope may be used in reading the thermometer. The instrument has been modified by Prof. Connell in substituting for the tube a small flask of highly polished brass or silver, into the neck of which is secured an exhausting syringe. The second of the two methods above referred to, by which the humidity of the air is ascertained, involves the determination of the temperature of evaporation; and the instrument used is the wet bulb thermometer or psychrometer invented by Prof. August of Berlin, and described in his work Ueber die Fortschritte der Hygrometrie (Berlin, 1830). It consists of two delicate thermometers placed near together. The bulb of one is covered with muslin, which is kept wet by water supplied from a vessel close by through capillary conduction. The instrument is placed in a light draught of air, and as evaporation goes on the mercury in the wet bulb thermometer sinks to a certain point; the temperature of both is then noticed. If the air was nearly saturated with moisture, the difference will be found to be very slight. The barometric pressure is observed at the same time, and data are thus afforded for calculating the elastic force of aqueous vapor in the air. The formula for this calculation, modified by Regnault, and the psychrometrical tables deduced from it, are given in the volume of tables referred to above, and are equally applicable to the estimation whether the dew point instrument or wet bulb thermometer be used. To render them more convenient, they have been converted by Prof. Guyot into English measures. The series also contains tables of the weight of vapors in a given space at different temperatures. The method by the wet bulb, though regarded as decidedly the most accurate means of determining the elastic forces of the vapor, and thence the humidity of the air, is still rendered somewhat uncertain in its results from the impossibility of keeping the wet bulb uniformly moist, and from other causes also.-The ultimate object of these hygrometrical investigations is, by enabling the meteorologist to ascertain at all times, in all localities, and at all accessible elevations, the true condition of the atmosphere as to moisture, to furnish him with accurate data for studying the laws which control its variations. Among the meteorological works which may advantageously be consulted, beside those already named, are Kömtz's Lehrbuch der Meteorologie, or the same translated by Charles Martius; Daniell's "Meteorological Essays;" and Blodget's "Climatology of the United States" (Philadelphia, 1857).

HYKSOS, or SHEPHERD KINGS, a pastoral Asiatic race who invaded and conquered Egypt at a remote period, computed by Bunsen at 2567 or 2477 B. C., by Wilkinson at 2240, 2173, or 2059 B. C., and by Lepsius at about 2000 B. C. The only detailed account of them that has reached us from any ancient writer is the following fragment of a lost work of the Egyptian Manetho (280 B. C.), quoted by Josephus in his reply to Apion: "We had once a king called Timæos, under whom, from some cause unknown to me, the Deity was unfavorable to us, and there came unexpectedly from the eastern parts a race of men of obscure extraction, who confidently invaded the country and easily got possession of it by force, without a battle. Having subdued those who commanded in it, they proceeded savagely to burn the cities, and razed the temples of the gods, inhumanly treating all the natives, murdering some of them, and carrying the wives and children of others into slavery. In the end they also established one of themselves as a king whose name was Salatis; and he took up his abode in Memphis, exacting tribute from both Upper and Lower Egypt, and leaving garrisons in the most suitable places. He especially strengthened the parts toward the east, foreseeing that on the part of the Assyrians, who were then powerful, there would be a desire to invade their kingdom. Finding therefore in the Sethroite nome a city very conveniently placed, lying eastward of the Bubastic river, and called from some old religious doctrine Avaris [or Abaris], he built it up and made it very strong with walls, settling there also a great number of heavy-armed soldiers, to the amount of 240,000, for a guard. Hither he used to come in the summer season, partly to distribute the rations of corn and pay the troops, partly to exercise them carefully by musterings and reviews, in order to inspire fear into foreign nations. He died after a reign of 19 years. After him another king called Baion reigned 44 years; after him another, Apachnas, 36 years and 7 months; then Apophis 61 years, and Jaunas 50 years and one month; last of all Assis, 49 years and 2 months. And these 6 were their first rulers, always carrying on war and desiring rather to extirpate the Egyptians. Their whole nation was called Hyksos, that is, shepherd kings; for hyk in the sacred language denotes king, and sos is a shepherd in the common dialect, and hence by composition HykThe before-named kings and their descendants were masters of Egypt for 511 years. After this a revolt of the kings of the Thebaid and the rest of Egypt took place against the shepherds, and a great and prolonged war was carried on with them. Under a king whose name was Misphramuthosis, the shepherds were expelled by him from the rest of Egypt after a defeat, and shut up in Avaris. Thutmosis, the son of Misphramuthosis, endeavored to take the place by siege, attacking it with 480,000 men. Despairing of taking it, he made a treaty with them that they should leave Egypt and

SOS.

astronomer, under whom she made great progress in geometry. She repaired to Athens near the close of the 4th century, and studied under Plutarch, the head of the Athenian school, who expounded to a small circle of disciples, one of whom was his daughter Asclepigenia, the Chaldean oracles and the secrets of theurgy. On her return to Alexandria, her talents, beauty, graceful eloquence, and modesty at once made her an object of admiration. She revived the languishing Neo-Platonic school of Plotinus, became its head, and expounded her doctrines before a numerous and enthusiastic auditory. Synesius of Cyrene, bishop of Ptolemais, acknowledged himself her disciple, and corresponded with her, addressing his letters ry porop But both as a pagan and as a philosopher she is said to have provoked the hostility of Cyril, bishop of Alexandria. Not only her lecture room was thronged, but she was consulted by the most considerable persons of the city, among others by the prefect Orestes, who was at constant feud with the bishop. The city was a prey to the violence of parties, and it was to the influence of Hypatia that Cyril attributed the refusal of Orestes to come to a reconciliation. "Certain persons, therefore," says the ecclesiastical historian Socrates, "of fierce and over-hot minds, who were headed by one Peter, a reader, conspired against the woman, and observed her returning home from some place; and having pulled her out of her chariot, they dragged her to the church named Cæsareum, where they stripped her and murdered her. And when they had torn her piecemeal, they carried all her members to a place called Cinaron, and consumed them with fire. This fact brought no small disgrace upon Cyril and the Alexandrian church." Cyril is accused by Theodoret of sanctioning this murder, but Cave regards it as incredible, though only on the ground of Cyril's general character. Hypatia was the author of two mathematical treatises, which are lost, and there remains from her only an astronomical table inserted in the manual tables of Theon. She is the heroine of one of Charles Kingsley's historical romances, which bears her name.

HYPERBOLA (Gr. vñeрßaλλw, to transcend), one of the conic sections, produced when the cutting plane makes a smaller angle with the axis of a right cone than is made by the side. The shadow of a globe on a flat wall, when part of the globe is further than the luminous point is from the wall, gives a hyperbola. Hyperboloids are surfaces generated by the motion of hyperbolas.

HYPERBOREANS, a legendary race, placed by the Greeks in the remote regions of the north, beyond the domain of Boreas or the north wind. They were unknown to Homer, and first appear in Hesiod and in the traditions connected with the temples at Delphi and Delos. The poets and geographers conceived of them as a pious nation, dwelling in perpetual sunshine, possessing abundant fruits, abstaining from the flesh of animals, and living for a thousand years

in innocence and peace, in the service of Apollo, delighted by dances and music. Humboldt finds in the legend "the first views of a natural science which explains the distribution of heat and the difference of climates by local causes, by the direction of the winds, the proximity of the sun, and the action of a moist or saline principle." The supposed location of the Hyperboreans changed with the progress of geograph ical knowledge. At first placed in the north at the sources of the Ister (Danube), they were transferred by some to the west when this river was supposed to proceed from the western extremity of Europe; while others transferred them to the extreme north of Europe, beyond the mythical Gryppes and Arimaspi, who themselves dwelt beyond the Scythians. The latter view at length prevailed; the character of the Hyperboreans as a sacred nation was lost sight of; and their name became only a geographical expression for the extreme north.

HYPERIDES, one of the 10 Attic orators, born probably about 395 B. C., died in Ægina in 322. He was a pupil of Plato in philosophy, of Isocrates in oratory, began his career as an advocate, and was an associate of Demosthenes as leader of the anti-Macedonian party. In 358 he and his son equipped two triremes at their own expense to join the expedition against Euboea. He displayed an equal interest in the patriotic cause on an embassy to Rhodes (346), in the expedition against Byzantium (340), as ambassador with Demosthenes to Thebes after the capture of Elatea by Philip (338), and after the battle of Charonea, when he proposed, by a union of the citizens, resident aliens, and slaves, to organize a desperate resistance to Philip. For his efforts on the last occasion he was prosecuted in an indictment for illegal proposition, but was acquitted. Of his defence there remain only the words: "The Macedonian army darkened my vision; it was not I that moved the decree, but the battle of Charonea." The affair of Harpalus (324) for the first time broke his friendly relations with Demosthenes, against whom he appeared as public prosecutor. On the report of Alexander's death (323), it was chiefly by his exertions that the confederacy was formed which brought about the Lamian war. He fled after the battle of Crannon to Ægina, and was pursued and put to death by the emissaries of Antipater. The ancient critics agree in extolling his genius, and commend him for almost every excellence of style. Until lately, however, only unimportant fragments of his orations were known to have been preserved. In 1847 A. C. Harris, an English resident at Alexandria, purchased at Thebes some fragments of papyrus written over with Greek, which were parts of the oration of Hyperides against Demosthenes on the charge of having been bribed by Harpalus. He published a facsimile of them in 1848. They were edited by Churchill Babington, with an introduction and commentary, in 1850. Another Englishman, Joseph Arden, procured at the same place and nearly the same time other

fragments of papyrus, which were found to contain a large part of his speech for Lycophron, prosecuted for adultery, and his complete ora tion for Euxenippus, charged with making a false report of the oracle of Amphiaraus. These were edited by Mr. Babington in 1853. Another traveller, Mr. Stodart, brought from Egypt in 1856 another collection of papyrus fragments, among which were a large part of the funeral oration on Leosthenes and the Athenian soldiers who perished in the Lamian war. This was published by the same editor in 1858. These works restore Hyperides to his place among the great Attic orators; and though his style does not equal that of Demosthenes in force, dignity, and careful preparation, its clearness, fluency, and eloquence justify the praises of the ancients. His orations have been republished in Germany by Böckh, Kayser, and others, and in Paris in Didot's Bibliotheca Græca. The funeral oration has been edited by Cobet (Leyden, 1858).

HYPERTROPHY (Gr. Tep, over, and Tpoon, nourishment), an excess of growth of a part without degeneration or alteration in the structure; the exact opposite to atrophy. Hypertrophy may depend on the excess of the materials of certain tissues in the blood; when this fluid contains habitually too much fat, there may be an abnormal increase of the adipose tissue; when one kidney cannot perform its functions, the other increases in size on account of the accumulation of its special materials in the blood; similar hypertrophy may thus be induced in other tissues, but there is no evidence that the muscles or nerves increase in bulk from the mere excess of their formative materials. Though an increased supply of blood is generally rather the consequence than the cause of excessive nutrition in a part, hypertrophy may arise from a mere increased circulation; this must be distinguished from the augmented bulk of long congested parts, in which there is not normal hypertrophy, but an addition of altered and inferior tissue. Hypertrophy is in most cases. dependent on a preternatural formative capacity in the part, sometimes congenital (as in the abnormal growths of fingers and toes, and even . entire limbs), but generally acquired. The most striking instances of acquired nutritive activity are seen in the nervous and muscular systems, consequent upon the excessive exercise of their functional powers. Excessive use of the brain in young persons of precocious intellect increases its growth beyond the capacity of the cranium, and from the consequent pressure upon the vessels may arise inactivity, imbecility, and even coma. Muscular hypertrophy is most often seen in the involuntary muscles, whose action is in some way impeded; thus stricture of the urethra or stone in the bladder, obstructing the exit of the urine and calling for extra exertion to expel it, causes hypertrophy of the muscular coat of the bladder; so it happens with the gall bladder when its ducts are stopped by calculi, and with the intestines when a stricture exists in any portion. Hypertrophy of the ventricles of the heart

is almost always dependent on pulmonary stagnation and obstruction, or narrowing of the cardiac orifices by disease of the valves, giving the organ double work to do, and increasing its activity, as in other muscles. (See HEART, DISEASES OF THE.) When any of the voluntary muscles are specially exercised, hypertrophy is observed in them, as in the arm of the blacksmith or the legs of a professional dancer; and such hypertrophied muscles generally cause an increased nutrition of the bones to which they are attached, and an enlargement of the points of origin and insertion. There are certain enlargements of glands, in which their proper tissue is increased without structural change, which unite physiological hypertrophy with pathological tumors, as in the case of the mammary, thyroid, and prostate glands. Certain tumors of the uterus contain only an excess of the normal muscular and fibrous tissues of the organ, and yet cannot be regarded as examples of hypertrophy, as they observe no regular growth, subserve no physiological purpose, and constitute a positive deformity and disease; such abnormal growths may exist upon a uterus itself hypertrophied from increased functional activity, and must not be confounded with the latter. Supernumerary parts, as additional fingers and toes and various outgrowths developed during foetal life, must in like manner be referred to local hypertrophy from excess of formative activity. Dr. Carpenter sees in this whole series of abnormal productions the operation of a similar power; that which in simple hypertrophy is confined to increasing the size of an organ by the development of new tissue according to the morphological type of the part, in the formation of supernumerary tissues also imparts to them an independent existence; on the other hand, while in ordinary hypertrophy the tissues in excess are incorporated into the affected organ, in the structure of a tumor the perfectly formed and independently growing tissues constitute a mass whose shape is determined more by surrounding conditions than by any tendency of their own-the formative power undirected by the normal morphological nisus. In malignant growths, the development of tissue stops short of the limit by which formative power produces the normal tissues of the body, and their vital endowments are not sufficient to resist the tendency to degeneration.

HYPOCHONDRIA (Gr. To, under, and xovopos, cartilage), a disease generally classed among neuroses, characterized by derangement of various organic functions, and accompanied by an habitual sadness, often bordering on despair, and a disposition to exaggerate every trifling symptom into a sign of dangerous malady. It constitutes the "spleen" of the English, and is in common language abbreviated into "hypo." It occurs principally in persons of melancholic temperament, and in those whose moral and intellectual faculties have received high and unnatural development; it is said to be common in proportion to the elevation of the human