A.—­Marine engines are of two kinds,—­paddle engines and screw engines.  In the one case the propelling instrument is paddle wheels kept in rotation at each side of the ship:  in the other case, the propelling instrument is a screw, consisting of two or more twisted vanes, revolving beneath the water at the stern.  Of each class of engines there are many distinct varieties.

112. Q.—­What are the principal varieties of the paddle engine?

[Illustration:  Fig. 22.]

[Illustration:  Fig. 23.]

A.—­There is the side lever engine (fig. 26), and the oscillating engine (fig. 27), besides numerous other forms of engine which are less known or employed, such as the trunk (fig. 22), double cylinder (fig. 23), annular, Gorgon (fig. 24), steeple (fig. 25), and many others.  The side lever engine, however, and the oscillating engine, are the only kinds of paddle engines which have been received with wide or general favor.

[Illustration:  Fig. 24.]

113. Q.—­Will you explain the main distinctive features of the side lever engine?

A.—­In all paddle vessels, whatever be their subordinate characteristics, a great shaft of wrought iron, s, turned round by the engine, has to be carried from side to side of the vessel, on which shaft are fixed the paddle wheels.  The paddle wheels may either be formed with fixed float boards for engaging the water, like the boards of a common undershot water wheel, or they may be formed with feathering float boards as they are termed, which is float boards movable on a centre, and so governed by appropriate mechanism that they enter and leave the water in a nearly vertical position.  The common fixed or radial floats, however, are the kind most widely employed, and they are attached to the arms of two or more rings of malleable iron which are fixed by appropriate centres on the paddle shaft.  It is usual in steam vessels to employ two engines, the cranks of which are set at right angles with one another.  When the paddle wheels are turned by the engines, the float boards engaging the water cause a forward thrust to be imparted to the shaft, which propels forward the vessel on the same principle that a boat is propelled by the action of oars.

[Illustration:  Fig. 25.]

114. Q.—­These remarks apply to all paddle vessels?

A.—­They do.  With respect to the side lever engine, it may be described to be such a modification of the land beam engine already described, as will enable it to be got below the deck of a vessel.  With this view, instead of a single beam being placed overhead, two beams are used, one of which is set on each side of the engine as low down as possible.  The cross head which engages the piston rod is made somewhat longer than the diameter of the cylinder, and two great links or rods proceed one from each end of the cross head to one of the side levers or beams.  A similar cross bar at the other end of the beams serves to connect them together and to the connecting rod which, proceeding from thence upwards, engages the crank, and thereby turns round the paddle wheels.