Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/374

containing case whereby a cord can be coiled several times round the axis for the purpose of setting the fly-wheel in motion. There is also attached to the rigid case, in the plane passing through the centre of gravity of the wheel at right angles to its axis, a thin flange of metal, which is called the bearing edge. The circumference of this flange is not a circle but a curvilinear polygon of sixteen or more equal sides. The object of mak ing it so is to pre vent the instrument from rolling like a wheel on the bearing edge when the fly wheel is rotating rapidly. A draw ing of the gyrostat, with a portion of the case removed to show the inside, is given in fig. 4. A represents the bearing edge upon which the whole balances. To suit certain experiments, a sharp conical steel point can be fixed

on to the end B by means of a bayonet joint.

Fig. 4.

The gyrostat is a most instructive instrument, and with it many interesting experiments can be performed. We can only mention a few taken from Thomson and Tait s Natural Philosophy, vol. i. Part I., where the mathematics of the whole subject will be found fully given.

1. Let the gyrostat be placed on a flat sheet of glass with its bearing edge in a vertical plane as represented in the figure. Neglecting translations, it has in that position clearly two free doms, one in azimuth and the other inclmational. Of these, the first is neutral and the second unstable when the fly-wheel is still ; but when it rotates rapidly the second will become stable, the first remaining neutral as before. When the fly-wheel is spinning rapidly, the persistence with which the gyrostat keeps the erect position is very remarkable. A blow from the fist on the side of the case is met by a strong resistance, the instrument being thrown into a state of violent tremor, which subsides, however, after a few seconds. If, while the fly-wheel is still rapidly spinning, a weight be hung on at B, the whole apparatus will, like the gyroscope, immediately begin to move round a vertical axis.

2. Let the gyrostat be supported on knife-edge gimbals at its lower end, and with the axis vertical. In this position it has two freedoms each unstable, without rotation of the fly-wheel, but with it both stable. A similar result is obtained by supporting the gyrostat on a universal flexure joint, constituting an inverted gyroscopic pendulum.

3. Let the gyrostat be supported on two equally long stilts tan gential to the bearing edge. In this position the two freedoms, one azimuthal the other inclinational, are both unstable without, but both stable with, rapid rotation of the fly-wheel.

4. Let the gyrostat be attached to a bar of wood by thrusting the bearing edge through a narrow slot in the middle of the bar, and then let it be suspended by strings attached to the ends of the bar, the bar being horizontal. By this we have the means of slinging the gyrostat bifilarly in four different ways, two with the strings parallel and two with them crossed, the gyrostat being alternately above and below the wooden bar. Each of these ways has four freedoms, which can be reduced to three by a third string in each case. A little consideration will show in each case the stable and unstable modes without and with rotation of the fly-wheel.

It only remains to mention a very interesting and intricate gyrostatic problem which has been lately suggested by Sir William Thomson. He supposes a string of gyrostats to be formed with their axes all in the same straight line, and each attached to the other by a small universal flexure joint of thin steel wire. If this string be disturbed after the fly-wheels have all been put into rapid rotation, a remarkable cork-screw motion passes along the wire. For particulars we must refer to a paper announced by Sir William Thomson in the R.S.E. Proceedings.

In addition to the references in the text the following will be found useful : Ast. Notices, vol. i. ; Comptes Rendus, Sept. 1852 ; Paper by Professor Magnus translated in Taylor s Foreign Scientific Memoirs, new ser., part 3, p. 210: Ast. Notices, vol. xiii. pp. 221-248; Theory of Foucaulfs Gyroscope Experiments, by the Kev. Baden Powell, F.R.S. ; Ast. Notices, vol. xv. ; articles by Major J. G. Barnard in Sillimans Journal, 2d ser., vols. xxiv. and xxv. ; E. Hunt on "Rotatory Motion," Proc. Phil. Soc. Glasgow, rol. iv. ; J. Clerk Maxwell, " On a Dynamical Top," Trans. R.S.E.,o. xxi. ; Phil. Mag., 4th ser., vols. 7, 13, 14 ; Proc. Royal Irish Academy, vol. viii. ; Sir William Thomson on " Gyrostat," Nature, vol. xv. p. 297 ; Price s Infinitesimal Calculus, vol. iv.

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GYTHIUM was an ancient Achaean town on the Laconian Gulf, south-west of the mouth of the Eurotas, near the site of the modern port Marathonisi. It lay opposite the island Cranae, at the foot of the fertile valley of the Gythius. On its coins the common types are Apollo and Heracles, the founders of the city. Heracles, the Phoenician god Melkart, points to an early connexion with Tyre. The Phcenicians maintained a great trade with the shores of the Laconian Gulf, and Aphrodite Migonitis, the Phoenician Astarte, had a temple at Migonium, the modern Marathonisi. Aphrodite and Asclepius also occur on its coins, and the latter had a temple in the city. A great port in the period of Phoenician intercourse, Gythium became a secondary town after the Dorian conquest, as is proved by the absence of early coins ; and it was only after the decay of Sparta that it again became an import ant city. It was the ordinary station of the Spartan fleet, and was considered the port of Sparta, from which it was distant about 30 miles. In the wars against Athens it was therefore exposed to frequent attacks. Tol- midas, the Athenian commander, burned it (455 b.c.). Later it was besieged unsuccessfully by Epaminondas (370 b.c.). It was strongly fortified by the tyrant Nabis ; but he was compelled by Flamininus to give up Gythium and other coast towns to the Achaean league (195 b.c.). When, soon afterwards, the whole country became a Roman province, Gythium had its own magistrates, o-Tparayoi and </>opo<, (see inscr. in Lebas, Voyage Archeol., 242, and Gott. Gel. Anz., 1865, p. 461). Augustus made it one of the twenty -four Eleuthero-Laconian towns. The existing ruins, called Paleopoli, are all of the Roman period. According to Strabo (p. 3G3), it had an artificial harbour, of which Col. Leake could see no trace. In the town was a well sacred to Asclepius, and at three stades distance was the stone Argos, where Orestes was relieved from his madness.

GYULA, chief town of the Trans-Tibiscan county of Be ke s, Hungary, is favourably situated on the Fehe r (White) K orb s, and has a well-built station on the Nagy-Varad (Grosswardein) and Eszek line of the Alfold-Fiume Railway, in 46 38 1ST. lat., 21 17 E. long. The outer ditch of the old ruined fortress, the tower of which serves as a prison, divides Gyula into two parts, named respectively Magyar- Yaros (Hungarian-town) and Ne met-Varos (German-town). Stone bridges facilitate communication over the canals, which flow from the K orb s, and intersect the streets. Among the principal buildings are Roman Catholic, Old United Greek, Lutheran, and Calvinist churches, and a Jews synagogue, also a fine castle with gardens, a county hall, an ancient Turkish bath, many handsome private residences, and several oil-mills. Gyula is, moreover, the seat of the county assemblies and administration, of a royal court of justice, of the assizes, and of a board of taxes, and has post and telegraph offices, and a savings-bank. The inhabitants are employed partly in industrial and commercial pursuits, partly in fanning and cattle-breeding. The fairs are much resorted to for the purchase of corn, horned cattle, sheep, pigs, wooden wares, and gall-nuts. The chief agricultural products of the surrounding country are wheat, barley, oats, and maize; there is also a good supply of garden fruit, grapes, and vegetables. In 1870 the population amounted to 18,495, consisting chiefly of Magyars, Wallachs, and Germans.