But of all the arrangements of direct action engines, that which admits of the greatest length of stroke in the smallest compass, and which, perhaps, is also the simplest, is the Oscillating Engine, invented, as we have already mentioned, by Mr. Witty. The very extensive adoption of this form of engine for steam vessels, is mainly owing to the example of Messrs. Penn and Son, of Greenwich, who for some years past have employed it in preference to all others; and, by their judicious mode of construction, added to the exquisite style of workmanship, have attracted such notice to the arrangement that most of the leading engineers have to some extent adopted it; and it bids fair to supersede most other forms.
The annexed cut represents an arrangement of this description of engine adopted by Messrs. Penn in a vessel on the Thames, which differs slightly from their ordinary management. The foundation or bed plate of the engine is composed of a strong cast-iron frame, extending across the vessel, and divided into three compartments; the two side ones being occupied by the cylinders, and the central one a forming the condenser; b is one of the cylinders, which is suspended upon hollow axes or gudgeons, working in bearings attached to the bed-plate; c c are wrought-iron pillars, supporting the top frame d, that carries the bearing of the main shaft, which is formed into three cranks, to one of which, e, the piston-rodfof the steam cylinder i is connected, whilst the central one, g, works the air-pump h by the medium of the connecting rod i. The steam from the boiler enters by the gudgeon which is nearest the vessel's side, and arrives at the slide case k by the hollow passage or belt m. By the slide valve the steam is admitted to the cylinder, and discharged from the same, and passing off by the midship gudgeon n, which forms the eduction passage, it enters the condenser; o is the hot well, and p the waste water-pipe. The principal difference between the arrangement described and Mr. Penn's ordinary arrangement, is that in the latter the air-pump stands vertically between the two engines, and that the slide valve case is placed at the back of the engine, that is to say, midway between the two gudgeons.
We shall conclude our account of the varieties in the form of marine engines adopted by different engineers, by a description of a rotatory engine recently invented by Mr. Peter Borrie, and which, we believe, has been applied to several vessels.