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HYACINTH 99 of Ms day in astronomical observations. His discovery of the isochronism of the cycloid was one of the most important in mathematics; and not inferior to it is the invention of the in- volution and evolution of cur.ves, and the es- tablishment of the proposition that the cycloid is its own evolute. He also, in his Horologium Oscillatorium, gives a method for finding the centre of oscillation, which was the first suc- cessful solution of a dynamical problem in which connected material points are supposed to act on one another. The difficulty of this subject is shown by the fact that Newton fell into an error in regard to it in attempting to solve the problem of the precession of the equinoxes. The question of the centre of os- cillation had been proposed by Mersenne in 1016, and although some cases had been solved on the principle of the centre of percussion, it was beyond the reach of any methods th'en known. Huygens was only a boy of 17 when the question was proposed, and could then see no principle by which it could be solved ; but when he published his Horologium Oscillato- rium in 1673, the principles which he assumed led to correct results in all cases. The two first theorems appended to that work state : 1, that if two equal bodies move in unequal cir- cles in equal times, the centrifugal forces will be proportional to the diameters of the circles ; and 2, that if the velocities are equal, the cen- trifugal forces will be in the inverse ratio of the diameters. To arrive at these conclusions required the application of the second law of motion (i. e., that the motion which a force gives to a body is compounded with the motion which it previously had) to the limiting ele- ments of the curve, in the manner in which Newton afterward demonstrated the theorems of Huygens in his Principia. Huygens's own demonstrations of these theorems were found after his death among his papers. In his treatise on the impact of bodies (De Motu Corporum ex Percussione), Huygens must have assumed the third law of motion, which New- ton afterward expressed by saying that " action and reaction are equal and opposite," by which we understand that the quantity of motion in the impact of bodies remains unchanged, one of the first grand principles in the doctrine of the conservation of force. His works were edited by 's Gravesande under the titles of Opera taria (2 vols. 4to in 1, Leyden, 1724) and Opera Reliqua (2 vols. 4to, Amsterdam, 1728). HYACINTH, a genus of liliaeea, containing several species, the most important of which is hyacinthus oriental^, a native of the Levant. This has an onion-like bulb, which throws up long, narrow-channelled leaves, from among which arises a scape bearing a raceme of bell- shaped drooping flowers ; the parts of the pe- rianth are united to about the middle, and the free portions reflexed ; flowers often very fra- grant, appearing in early spring. This being one of the florists' flowers, great changes have been produced in it by cultivation ; the size of the flower cluster has been greatly increased, the flowers are semi-double and double, and there is a great variety of colors and tints, from pure white, through various shades of red and blue, to nearly black. The number of named varieties is very large, and includes not only self-colored ones, but double and single kinds, with flowers variously striped and sha- ded. The bulb growers near Haarlem in Hol- land supply the world with hyacinths, which - form a large share of what are imported under the name of "Dutch bulbs." The eminence of the Dutch florists in the culture of this and other bulbs is in part due to a favorable soil and climate, and in part to the patient care given to their cultivation ; these, with the low price of labor, have enabled them to hold a monopoly of bulb growing. Near Haarlem over 100 acres of land are annually devoted to hyacinths; the soil is a mixture of sand and alluvium, and permanently supplied with the requisite moisture. New varieties are obtained by sowing seed, and it is necessary to cultivate the seedlings for six years before their real Hyacinth Bulb and Section. merit can be decided upon. Established va- rieties are multiplied from the small bulbs which form at the base of the larger ones ; a bulb will naturally produce several of these, and the cultivators increase the number by wounding and cutting the bulb in various ways. The small bulbs are carefully cultiva- ted until of a proper size for market ; in or- der to increase its size as rapidly as possible, the bulb is not allowed to exhaust its strength in producing flowers, but the flower stem is cut away as soon as it appears. Millions of bulbs are annually imported into this country and England, and large quantities go to other countries. The best are imported by the deal- ers direct from the growers; it is only the poorer bulbs, from which the finer ones have been selected, that are usually offered at auc- tion. The diflferent varieties are put up in bags' of heavy paper, with an abundance of the hulls of buckwheat, and the bags are packed in cases. The heaviest bulbs, which show no signs of decay by being soft at the top, are