Page:The New International Encyclopædia 1st ed. v. 18.djvu/246

SIPHON. 202 SIREN. fill the tube and flow through the lower end. In this way it is possible to draw off the contents of casks, tanks/and other receptacles with great facility, unless the bend of the tube is more than 33 feet above the surface of water or a liquid of equal density, or 30 inclies in the case of mer- cury, in which ease the atmospheric pressure is not sufficient to support a column of liquid of such height. The reason can readily be seen, since a column of water 33 feet in height weighs as much as a column of atmosphere of equal cross-section, and consequently the effective pres- sure would be downward instead of up. that of the liquid being greater than that of the atmos- phere. The lower the outside tube below the surface of the liquid the more rapid will be the flow, which will continue until the level of the liquid either sinks below that of the outlet or air finds its way into the tube. Often a pump is connected with the siphon to remove the air, or in the case of a small tube such as would be used with a cask the suction of the mouth may prove sufficient. SI'PHONOPH'ORA (Neo-Lat. nom. pi., from Gk. <ri0wKo06pos, fsiplionophoros, carrying tubes, from al(t>av, siphon, pipe, tube + (pipeiv. phereiii, to carry). An order of the cfelenterate class Hydrozoa. They are the so-called compound hy- droids, living in free-swimming colonies, consist- ing of poljiuorphie individuals, or 'zooids" — that is, organs with a strongly marked individuality, but all more or less dependent on one another and originating from a common ctenosarcal tulie. In Physalia there are four kinds of zooids — i.e. (1) locomotive, and (2) reproductive, with (3) barren medusa buds (in which the pi-oboscis is wanting), which, by (heir contractions and dila- tations, impel the free-swimming animal through the water; in addition. Iheve are (4) the feeders, a set of digestive tubes which nourish the entire colony. The upper end of the eoenosareal tube is usually closed by a float, very large in Phy- salia. This float is filled with air, acts as a hy- drostatic apparatus, and enables the colony to maintain a vertical position in the water. See Portuguese Man-of-War; also Colored Plate of Medus.1: and Siphonopiiora. SIB-DARYA, ser'diir'ya. A river and terri- tory of Russian Turkestan. See Str-Darta. SIREN (Lat. siren, from Gk. <Teipiv, seiren, siren ; probably connected with <rvpiy^, syrinx, pipe, Skt. svar, to sound ; hardly akin to Gk. ffetpi, seira, cord). In Greek legend, one of sev- eral .sea-maidens with voices of such sweetness that all who heard w-ere drawn to them, only to meet death, so that the shores of their lovely island were covered with bones and corpses. In the Odi/ssei/ Circe gives Ulysses advice by which alone the hero passes in safety. He stopped the ears of his companions with wax. that they might not be turned from their rowing, while he caused himself to be firmly bound to the mast so that he might hear the songs without danger. They also figure in the voyage of the Argonauts, who only escaped because of the superior charms of the song of Orpheus. Later legend represented that, once successfully resisted, they were doomed. An- other legend connected them with the rape of Persephone. Here they were said to have grieved excessively at the loss of their friend and sought for her over land and sea. In this aspect they are common on tombstones, apparently as mourn- ers, often with disheveled hair. They are repre- sented in art at first as birds, with female beads, but more and more the human element predomi- nates, until there is little of the bird left but the wings and legs. The type seems connected with the representations of the souls of the dead in the form of birds. Consult Weicker. Der Heelcn- vogcl in dcr alien Litteratur und Eunst (Leipzig, IrtOS). SIREN. An instrument for the production of musical sounds in such a manner as to enable us to determine the number of vibrations which produce a given sound, or, in other words, the pitch. In the simplest form of siren there is a revolving disk wdiieh is pierced with a series of holes arranged in a circle whose centre is the centre of the disk. If air forced through a tube from a bellows or other source of pressure strikes the disk at a point which is passed by the holes in their revolution, a series of sounds will be produced by the successive puff of air escaping through these openings. While the disk revolves slowly, the ear distinguishes these successive puffs; but when the revolutions are more nu- merous than about ten per second, the successive puffs cannot be distinguished, and the recurrent soimds are merged into a uniform note, whose pitch rises (i.e. it becomes more and more shrill) the faster the disk revolves. Such an instru- ment works well when driven by water instead of air. What it shows is that musical sounds con- sist of the repetition, at equal and very small intervals of time, of some definite noise. By turning the disk by means of a train of wheels, so as to give it a definite rate of rotation, the lunnber of such repetitions per second necessary for the production of a given musical note may be measured. The siren invented by Cagniard de la Tour in 1819 is better adapted for such a purpose, as it registers the number of revolutions per second. In principle it is identical with the simpler in- .strument just described; but the details of its construction are different. It consists essen- tially of two circular disks, the upper of which CR088 SECTION OF SIREN. is free to revolve upon the lower, being pivoted at A and b. In each a series of holes is cut, ar- ranged at equal distances in a circle about its axis. Through the holes in the lower (fixed) plate, streams of air are admitted from a reser- voir. T>. connected with a bellows, and pass through the corresponding holes in the upper (movable) plate, when the pair of holes are