A.—­A single engine is no doubt more liable to stick upon the centre than two engines, the cranks of which are set at right angles with one another; but numerous paddle vessels are plying successfully that are propelled by a single engine, and the screw offers still greater facility than paddles for such a mode of construction.  In the screw engine referred to, as the cylinder is laid upon its side, there is no unbalanced weight to be lifted up every stroke, and the crank, whereby the screw shaft is turned round, consists of two discs with a heavy side intended to balance the momentum of the piston and its connections; but these counter-weights by their gravitation also prevent the connecting rod and crank from continuing in the same line when the engine is stopped, and in fact they place the crank in the most advantageous position for starting again when it has to be set on.

642. Q.—­Will you explain the general arrangement of the parts of this engine?

A.—­The cylinder lies on its side near one side of the vessel, and from the end of the cylinder two piston rods extend to a cross head sliding athwartships, in guides, near the other side of the vessel.  To this cross head the connecting rod is attached, and one end of it partakes of the motion of the cross head or piston, while the other end is free to follow the revolution of the crank on the screw shaft.

643. Q.—­What is the advantage of two discs entering into the composition of the crank instead of one?

A.—­A double crank, such as two discs form with the crank pin, is a much steadier combination than would result if only one disc were employed with an over-hung pin.  Then the friction on the neck of the shaft is made one half less by being divided between the two bearings, and the short prolongation of the shaft beyond the journal is convenient for the attachment of the eccentrics to work the valves.

644. Q.—­Will you enumerate some of the principal dimensions of this engine?

A.—­The bottom frame, on which also the condenser is cast, forms the base of the engine:  on one end of it the cylinder is set; on the other end are the guides for the cross head, and in the middle are the bearings for the crank shaft.  The part where the cylinder stands is two feet high above the engine platform, and the elevation to the centre of the guides or the centre of the shaft is 10 inches higher than this.  The metal both of the side frames and bottom flange is 1-1/4 inch thick.  The cylinder has flanges cast on its sides, upon which it rests on the bottom frame, and it is sunk between the sides of the frame so as to bring the centre of the cylinder in the same plane as the centre of the screw shaft.  The opening left at the guides for the reception of the guide blocks is 6 inches deep, and the breadth of the bearing surface is 11 inches.  The cover of the guides is 8 inches deep at the middle,