nate of lime. In the carboniferous age evaporating pans where salt water precipitated its solid contents existed in Nova Scotia, Michigan, Virginia, and Arizona, and later in the triassic in the region now occupied by the Llano Estacado and in the Indian territory. In all these localities proof is abundant that the strata of gypsum are precipitated sediments, and that the theory which attributes the formation to the action of acid springs is a fallacy.—The uses of gypsum in the arts are varied and important. When calcined, its combined water is driven off. If now ground to powder and again mixed with water, this water recombines with it, and the mass becomes first plastic, then solid, and takes the form of any model into which it may have been poured. This property of gypsum has many applications in the arts. It makes the most convenient of mortars, and was extensively used by the ancient inhabitants of Mexico as well as those of Egypt in their masonry. The use of gypsum in the formation of plaster casts is too common and well understood to require special notice. When mixed with glue water, plaster of Paris is converted into stucco. If mixed with a solution of borax, alum, or sulphate of potash, and subsequently rebaked and powdered, and again mixed with a solution of alum, it forms a hard cast which takes a high polish. This composition is called Keene's cement if made with alum, Parian if with borax, and Martin's if with potash. In the preparation of plaster of Paris, the gypsum rock is ground between buhr stones until it is reduced to a fine powder. This is calcined by being heated in kettles or stills, the escaping water producing a movement like ebullition. As calcined plaster absorbs moisture from the atmosphere, it should be prepared as wanted, or carefully protected from dampness. Gypsum is sometimes used for the glazing of porcelain. But the principal consumption of it is as a fertilizer for soils. Sulphate of lime enters into the composition of grasses, potatoes, turnips, &c., and these cannot flourish in soils entirely free from it. Its potency, however, is probably due in a far greater degree to its action in fixing volatile and escaping carbonate of ammonia. When this comes in contact with sulphate of lime, double decomposition takes place, carbonate of lime and sulphate of ammonia being formed. Its value as a fertilizer may be readily tested by distributing a quantity of it in a narrow line across a meadow. Where the plaster has fallen, the grass will frequently be so much stronger and greener, that the difference may be seen even at a considerable distance. Gypsum is not unfrequently mingled with, and sometimes shades into, anhydrite, the anhydrous sulphate of lime. Both pass under the name of plaster, the anhydrite being called hard and gypsum soft plaster. The uses to which they are applied are the same.—The commerce in gypsum in the United States amounts to about $1,000,000 per annum, almost equally divided between the miner and manufacturer, and between three districts: the Atlantic coast, where the material is derived from Nova Scotia, and the states of New York and Michigan, where it is indigenous and abundant. The importation of gypsum into the United States from Nova Scotia and New Brunswick in 1873 was 118,280 tons, valued at $117,828. The annual production of gypsum in New York and Michigan may be estimated at about 100,000 tons each.
GYROSCOPE (Gr. γυρός, a ring, and σκοπεῖν, to examine), a name applied to various instruments designed to illustrate the phenomena of rotation. The most curious and generally interesting form of gyroscope, rightly named “mechanical paradox,” although its principle was discovered long before its first construction, consists essentially of a disk revolving on pivots within a ring, having on the line of prolongation of its axis, on one side, a bar or spur with a smooth notch beneath to receive the hard smooth point of an upright support. Thus placed, when the disk is not turning, the whole falls, of course, like any heavy body unsupported. Rotate rapidly by unwinding a string, set on the support, but uphold the opposite side of the ring; no peculiar movement then occurs. But if while the disk is rapidly turning, the bar being on the support, the opposite side be set free, the whole, instead of falling, as would be expected, commences a steady revolution in a horizontal circuit about the point of support, moving more rapidly as the primary rotation is expended, and sinking, at first imperceptibly, then more rapidly, until in from one to three minutes it comes to rest.
Gyroscope.
Mathematical analysis shows that when set free, it continually falls and rises, but this motion is not visible. The disk started with its axis in or below the horizontal never rises, without aid, above its first position. Started with high speed above the horizontal, it may rise, and if its connection with the support allow, as when this is by a ball and socket, it may even ascend to a vertical position, and spin as a top. Arrested in its travelling movement, it always descends; hastened, it rises. Checked in any part, it inclines in the direction of that part. In the form now given, the travelling or orbital movement is always in the
