A.—­The engine occupies little space, consists of few parts, is easily accessible for repairs, and may be both light and strong at the same time.  In the case of large engines the crank in the intermediate shaft is a disadvantage, as it is difficult to obtain such a forging quite sound.  But by forging it in three cranked flat bars, which are then laid together and welded into a square shaft, a sound forging will be more probable, and the bars should be rounded a little on the sides which are welded to allow the scoriae to escape during that operation.  It is important in so large a forging not to let the fire be too fierce, else the surface of the iron will be burnt before the heart is brought to a welding heat.  In some cases in oscillating engines the air pump has been wrought by an eccentric, and that may at any time be done where doubt of obtaining a sound intermediate shaft is entertained; but the precaution must be taken to make the eccentric very wide so as to distribute the pressure over a large surface, else the eccentric will be apt to heat.

437. Q.—­Have not objections been brought against the oscillating engine?

A.—­In common with every other improvement, the oscillating engine, at the time of its introduction, encountered much opposition.  The cylinder, it was said, would become oval, the trunnion bearings would be liable to heat and the trunnion joints to leak, the strain upon the trunnions would be apt to bend in or bend out the sides of the cylinder; and the circumstance of the cylinder being fixed across its centre, while the shaft requires to accommodate itself to the working of the ship, might, it was thought, be the occasion of such a strain upon the trunnions as would either break them or bend the piston rod.  It is a sufficient reply to these objections to say that they are all hypothetical, and that none of them in practice have been found to exist—­to such an extent at least as to occasion any inconvenience; but it is not difficult to show that they are altogether unsubstantial, even without a recourse to the disproofs afforded by experience.

438. Q.—­Is there not a liability in the cylinder to become oval from the strain thrown on it by the piston?

A.—­There is, no doubt, a tendency in oscillating engines for the cylinder and the stuffing box to become oval, but after a number of years’ wear it is found that the amount of ellipticity is less than that which is found to exist in the cylinders of side lever engines after a similar trial.  The resistance opposed by friction to the oscillation of the cylinder is so small, that a man is capable of moving a large cylinder with one hand; whereas in the side lever engine, if the parallel motion be in the least untrue, which is, at some time or other, an almost inevitable condition, the piston is pushed with great force against the side of the cylinder, whereby a large amount of wear and friction is occasioned.  The trunnion bearings, instead of being liable to heat like other journals, are kept down to the temperature of the steam by the flow of steam passing through them; and the trunnion packings are not liable to leak when the packings, before being introduced, are squeezed in a cylindrical mould.