117. Q.—­Are all marine engines condensing engines?

A.—­Nearly all of them are so; but recently a number of gunboats have been constructed, with high pressure engines.  In general, however, marine engines are low pressure or condensing engines.

118. Q.—­Will you now describe the chief features of the oscillating paddle marine engine?

A.—­In the oscillating paddle marine engine, the arrangement of the paddle shaft and paddle wheels is the same as in the case already described, but the whole of the side levers, side rods, cross head, cross tail, and connecting rod are discarded.  The cylinder is set immediately under the crank; the top of the piston rod is connected immediately to the crank pin; and, to enable the piston rod to accommodate itself to the movement of the crank, the cylinder is so constructed as to be susceptible of vibrating or oscillating upon two external axes or trunnions.  These trunnions are generally placed about half way up on the sides of the cylinder; and through one of them steam is received from the boiler, while through the other the steam escapes to the condenser.  The air pump is usually worked by means of a crank in the shaft, which crank moves the air pump bucket up and down as the shaft revolves.

119. Q.—­Will you give an example of a paddle oscillating engine?

A.—­I will take as an example the oscillating engines constructed by Messrs. Ravenhill & Salked, for the Holyhead Packets.  Fig. 27 is a longitudinal section of this vessel, showing an engine and boiler; and fig. 28 is a transverse section of one of the engines, showing also one of the wheels.  There are two cylinders in this vessel, and one air pump, which lies in an inclined position, and is worked by a crank in the shaft which stretches between the cylinders, and which is called the intermediate shaft.  A A, is one of the cylinders, B B the piston rod, and C C the crank.  D is the crank in the intermediate shaft, which works the air pump E. There are double eccentrics fixed on the shaft, whereby the movement of the slide valves is regulated.  The purpose of the double eccentrics is to enable an improved arrangement of valve gear to be employed, which is denominated the link motion, and which will be described hereafter.  I I are the steam pipes leading to the steam trunnions K K, on which, and on the eduction trunnions connected with the pipe M, the cylinders oscillate.

120. Q.—­By what species of mechanism are the positions of the paddle floats of feathering wheels governed?

A.—­The floats are supported by spurs projecting from the rim of the wheel, and they may be moved upon the points of the spurs, to which they are attached by pins, by means of short levers proceeding from the backs of the floats, and connected to rods which proceed towards the centre of the wheel.  The centre, however, to which these rods proceed is not concentric with the wheel, and the rods, therefore, are moved in and out as the wheel revolves, and impart a corresponding motion to the floats.  In some feathering wheels the proper motion is given to the rods by means of an eccentric on the ship’s side.  The action of paddle wheels, whether radial or feathering, will be more fully described in the chapter on Steam Navigation.