PADDLE-FISTI, Polgodon pion, a fish inhabitino. the Mississippi river and .its tributaries. It is about 5 ft. long. without scales, and lms a long. bony snout of about the length of the body, with which it•plows or digs up the mud of the bottom in search of food. IL has a dark bluish back and a whitish belly.
PADDLE-WHEEL—one of the appliances in steam-vessels by wLiell the power of the engine is made to act upon the water and produce locomotion—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 use of paddlewheels in conjunction with steam as a motive-power dates from about the commencement of the present century, but the employment of the paddle-wheel itself is as ancient as the time of the Egyptians. A speci men is also known to have been tried in Spain in the 16th century.
The Fig.. shows the usual form of paddle-wheel, that ealleil the radial. in which the floats are fixed. It will be seen that a certain loss of power is in volved, as the full force of the engine on the water is only experienced when the float is vertical, and as on entering and leaving the water the power is mainly employed in depressing or lift ing the particles of water. This objec tion laas great force It 1110 moment of starting, or when progress is very slow, as is illustrated by the small 'power a paddle steamer evinces when trying to tug a stranded vessel off a sandbank; but when, in full progress, the action is less impeded by this circumstance, the water in front of the wheel being depressed, and that abaft being thrown into the form of a wave. The extent of the
immersion much influences the economy of power, as will be readily understood if the con sequences of immersion up to the center of the wheel be imagined. An immersion somewhat over the top of the lowest float is about the most advantageous, and in order that the floats may be as nearly as possible vertical when they strike the water, it is advisable to give the wheel as large a diameter as possible, and to place the axis at the highest availTtble point in the vessel.
To overcome the drawbacks to the radial wheel, Elijah Galloway patented, in 1829, the feathering paddlewheel, in which the floats are mounted on axes, and are connected by rods with a common center. which revolves upon a pin placed eccentrically to the axis of the paddle-wheel. By this method the floats are kept, while immersed, at right angles to the surface of the water. So long as the water is smooth the gain is great, con sequently feathered floats are much used in river-steameis; but for ocean-steamers the liability to derangement, perhaps at a critical period, is a great objection to their use.
The paddle-wheel, in revolving, imparts both a for aid velocity to the vessel and a backward velocity to the water. The latter is called the slip, and sometimes bears a very large and wasteful proportion to the former. The absolute velocity of the paddle floats is equal to the sum of the slip and the forward motion of the ship, so that the wheel always revolves faster than the ship makes way.