Propeller

(pro-pel'lur.) (Mech) A name generally applied to any appliance set in motion by some mechanical power in ve els or ships, which causes them to advance by the resistance of the water itself. Oars and sails were anciently the only propellers used; but when the steam-engine was applied to the purposes of navigation, paddle-wheels, and later the screw, became generally used as propellers. A paddle-wheel is a skeleton wheel of iron, on the outer portion of whose radii flat boards, called floats or paddles, are fixed, which beat upon the water, and produce, continuously, the same effect as is given, in an intermittent manner, by the blades of oars. The screw-propeller consists of two or more blades, somewhat like the vanes of a windmill, fixed on an axis running parallel to the keel of a ve el, and revolving under water at the stern - a water-tight opening being made for its axis, or shaft, just inside the stern-post. It is driven by a steam engine, placed within the ship; and by screwing itself, as it were, into the water, pre es the ve el before it. The threads of this screw must be greatly deeper than if it worked in metal or wood; and hence the blades are made of considerable width, and of a length nearly sufficient to reach from the keel to the surface of the water. The latter is prevented from entering where the screw-shaft pa es out at the stern,by a stuffing box. From experiments made on the subject, there is reason to believe that the screw is rather more advantageous than the paddle-wheels, when the ve el is deep in water; but the contrary when the immersion is light, or at a medium. Screw-ships are not so weli adapted to go head to wind as those with paddles; but they are more conveniently used as sailing-ve els, should circumstances require them partly,or altogether, to depend on the wind; and they are le exposed to the violent effects of a storm or heavy sea. A new propeller, invented by Mr. Frank G. Fowler, of Bridge port, and consisting of a submerged wheel that rotates on a vertical shaft, has been put lately into practical use on several ve els of considerable size, on the Delaware river and elsewhere, with gratifying succe . It dispenses with the rudder, and is capable of propelling the ve el to which it is attached either forwards, backwards, or sideways; and of causing it to turn in either Pro pert i us. direction upon SEXTUS its own AURELIUS, centre. (pro-pur'shús,) a Roman poet, B. in Umbria, 50 B. c. He enjoyed the patronage of Mæcenas and the friendship of Ovid, and has left to us four books of Elegies, written in imitation of the Greek style. Property, (prop'ür-te.) (From L. proprius, proper.] A peculiar virtue, quality, or attribute which is in he rent in, or naturally e ential to, some things exclusive of all others. Thus, color is a P. of light; extension, figure, divisibility, and impenetrability areP. of bodies, .-(Log.) A predicable which betokens something e entially conjoined to the e ence of the species. (Law.) Unrestricted and exclusive right to a thing. Prophet, (profèt.) [From Gr. prophètès, I say beforehand.] In general, one who foretells events. Among the canonical books of the Old Testament are the writings of sixteen prophets, four of whom are denominated the " greater prophets," viz., Isaiah, Jeremiah, Ezekiel, and Daniel, who were so called from the length or extent of their writings, which exceed those of Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah and Malachi. Jonah, the earliest of the prophets, lived about 800 B. c.; and Malachi, the latest, about 400 B. c. Prophylactic, (prof-e-luk'tik.) [FromGr. prophylaktikos, precautionary.) (Med.) A preservative or preventive. Propitiation, (-pish-e-a's hun.) [From L. propitiatio, an appeasing.) (Theol.) The atonement or sacrifice offered toGod to a uage his wrath, and render him propitious. Among the Jews, there were both ordinary and public sacrifices, offered by way of thanksgiving; and extraordinary ones, offered by particular persons guilty of any crime,by way ofP.-A feast among the Jews, celebrated on the 10th of the month Tisri, in commemoration of the divine pardon proclaimed to their forefathers through Moses, who, as God's agent, remitted the punishment due to the crime of their worshipping the golden calf. Propitiatory, (-pish'e-a-to-re.) [From L. propitiatorium, a means of atoning for.] (Jewish Hist.) The Mercy-seat; the cover or lid of the Ark of the Covenant, lined inside and outside with plates of gold. Propolis, (pro'po-lis.) (Gr., literally, in front of a city. ] A thick, odorous wax-like substance, used by bees for stopping the holes and crevices in their hives, to prevent the entrance of cold air, . Pro pont is, (pro-pon'tis.) (Anc. Geog.) The ancient name of the Sea of Marmora. Proportion, (-pōr'shūn.) [L. proportio.] In a general sense, the relation of any one thing to another.(Math.) An equality of ratios; but the term is sometimes improperly used for ratio. The arithmetical ratio between two quantities consists in their difference thus the arithmetical ratio between 6 and 4 is 6-4, or 2. And four quantities constituto an arithmetical P. when they form two ratios having equal differences; thus 8 is to 6 as 4 is to 2, or, more briefly, 8: 6:: 4: 2; because 8-6-1-2. The geometrical ratio between Pro nitz, two quantities consists in their quotient; thus, the geometric al ratio between 4 and7 is. And four quan-Prostate tities constitute a geometrical P. when they form equal ratios, that is, two fractions having equal quotients; thus 8: 4:: 6: 3, because - Quantities form an harmonic P. when, of three numbers, the first is to the third as the difference of the first and second is to the difference of the second and third. Thus 2, 3, 6 are an harmonic al P.; for 2 is to 6 as 1 is to 3. The rule ofP. enables us, when three terms of geometrical P. are given, to find the fourth. If more gives more, the P. is said to be direct; if more gives le , and vice vers a, it is Prostration, said to be inverse-but improperly, as only the mode of stating is affected by these circumstances.- (Fine Arts.) The proper relation of the measure of parts to each other and to the whole. Prostyle, Proposition, (prop-o-zish'ün.) [From L. propositio, a pointing out.) (Rhet.) Anything stated or affirmed for argument or illustration.-(Log.) That part of an argument in which some quality, negative or positive, Prot agoras, is attributed to a subject; as, " man is fallible." (Math.) A statement in terms of either a truth to be Proteaceæ, demonstrated, or an operation to be per for me d. It is called a problem when something is to be done; and a theorem when something is to be proved. [s. 735]