PROPELLER. In works upon the construction of machinery the word propeller is used to indicate the arrangement bi• means of which motion is given to a carriage bearing a portion of t le working gear required to traverse regularly iu a horizontal direction ; or to indicate a peculiar mechanism, set in motion by some mechanical power in weasels or ships which causes the latter to advance by the resist. ance offered by the water. The former class of propellers are most frequently employed in tools, and metal-working machinery, and con eist of a Geed shaft bearing a screw thread, gearing into a nut work ing upon guides, so as only to be able to move Literally. The latter are more numerous, and may be said to consist of oars, sails, paddle wheels, and screw-propellers. Sometimes the locomotive engine is used to propel loads on land; but this is so manifestly a modification of its tractive powers that it may for the present at least be neglected. Attention will in the following article be principally called to the propellers used for steam navigation, unquestionably tho must impor tant machines of this description now in use Until the application of the steam engine to the purposes of naviga tion, oars and sails were tho only propellers used. Oars are, in fact, levers worked by hand against a fulcrum fixed on the boat itself, and causing the latter to advance by the reaction of the water upon the boat. Saila are propellers by reason of the resistance they offer to the movement of the wind, which thus exercises a motive power upon the vessel by Ha transmission to the hull, through the masts. Accordingly, therefore, as the mils are fixed with regard to the longitudinal axis of the ship, do they cause it to advance either in the line of tho keel, to drop to leeward, or to turn on its vertical axis, with a velocity depending upon the size of the sails, the velocity of the movement of the wind, and upon the shape of the vesseL These various condi tions come more especially under the province of ship-building, or of navigation, and are therefore only alluded to here incidentally.
The use of wheels bearing floats working in the water by the side of the bones to which they were fixed, had been known from a very remote period ; but it was not until the steam engine was applied as a motive power to them that they were commonly introduced as pro pellers. At the present day the majority of steam-boats designed to work in comparatively still water, and oven the majority of the vessels used in the deep-sea packet service, are provided with these organs of locomotion. It is found, indeed, that paddle-wheel steamers can attain a greater economical velocity than screw steamers hitherto have done ; and the noise and vibration of the screw renders the class of vessel to which it is applied so uncomfortable, that it cannot be considered applic able to passenger traffic. Upon some rivers and large artificial canals, the waves caused by paddle-wheels are thought to be so injurious to the banks, that screw-propellers are substituted for them; but the agitation of the water below the surface produced by the revolution of the screw seems to be nearly as mischievous as thesurface waves caused ,by the paddles; so that the most important advantage possessed by the screw propeller over the paddle-wheel, for narrow water-courses, finally resolves itself into the smaller width it allows to be given to the In deep-sea-going vessels, which do not carry passengers, the 'advantages of the screw are so great that nearly all the new traffic steamers are fitted with propellers of that description; • and the company founded by Air. Smith, in 1836, which built the Archimedes,
has indeed already changed the whole condition of our commercial marine.
When paddle-wheels are used as propellers, they are placed upon a horizontal shaft or axis, and in front of the centre of gravity of the vessel; but there does not yet seem to be any uniformity of practice amongst constructors with regard to the precise position of those parts of the machinery ; and even in the case of some of the canal beets used in Belgium the wheels are fixed at the stern, without any apparently disadvantageous results. In sea-going boats the wheels are, however, always placed on the outside, and about the middle of the boat ; they are keyed upon a horizontal shaft bearing upon tho solid framework of the slip's side, and alined always upon an external seat. ing, carried by the framework of the paddlcebox. The paddlo.wheel shaft is continuous through the vessel when only one engine is used, and it is made in two pieces, with a movable clutch or coupling joint, when two engines arc required, as is the case in all sea-going vessels. Motion is communicated to the shafts, either by means of cranks con nected with the ends of balance beams, or with the piston heads of direct acting, or of oscillating, engines; and the position of the cranks toward, one another is made such that the piston of one engine ehalt be able to exorcise its full power, when the other is over its dead points. The wheels themselves consist of a series of radial arms, con nected with one another by means of concentric rings (both the arms and circles being made of wrought iron), and upon the arms aro fastened the floats, or the boards, whose revolution in the water pro duces the propulsive action. Very warm discussions have taken place with respect to the Ions of power which necessarily attends the ordinary form of wheel, in which the floats are fixed radially to the wheel ; and ninny ingenious contrivances have been proposed for the purpose of causing the Boatel to enter and to leave the water in a perpendicular direction. For vessels making short runs, the feathering paddles appear to produce satisfactory results; but it seems to be more than questionable whether the complication of the machinery, the additional weight of the wheels, and the con sequent increase of power required to move them, do not more than compensate for the theoretical advantage gained by diminishing the slip observable in ordinary paddles. It is to be observed also, that numerous modifications of the form of the float beards have been introduced in order to diminish their inconveniences; and Field's wheel, in which the boards are divided into narrow slips, arranged in cycloidal curves, so as to enter the water at the same place in immediate suecension, seems to bo the best of its kind. Morgan's feathering wheel is one of the most successful of those engines; but after all the effective gain is very small, even with it. Gene rally speaking, the proportion of the useful effect of paddle-wheel engines to the power exerted is considered to be as to 1.0 in the case of radial paddles; in the case of the feathering paddles it hardly exceeds 0-66 to 1.00, although at times it may rise to to 1-000.