With the side-lever engines the difficulty of driving the paddle-wheels at the required speed was overcome by the introduction of the gearing wheel, and this contrivance was applied also to the engines for driving the screw propeller. As the piston speed was increased it became possible to connect the crank shaft direct to the screw shafting. The engines used for driving paddle-wheels were either inclined or vertical, but after the introduction of the screw propeller for warships, the engines were built of the horizontal type and this method remained in vogue for about thirty years. The difficulty of working in the confined space was met by Mr. Penn with the trunk engine, with which he achieved a still greater success. His equipment of the warships _Arrogant_ and _Encounter_ with trunk engines in 1847 so satisfied the Admiralty that engine power for driving screws could be placed so far below the water-line as practically to be safe from an enemy’s shot, that altogether he applied them to no fewer than two hundred and thirty vessels, from a gunboat requiring 20 h.p. to such ships as the _Sultan_, 8,629 h.p., and the _Neptune_, 8,800 h.p. This invention, curiously enough, helped in the retention of the sailing power, by leaving the decks unencumbered by engine houses and paddle-boxes, so that when the engines were not in use the vessels could be kept under sail only, to gladden the hearts of the adherents of the old school. It also hastened the abolition of sail, for it showed that the machinery could be placed below the water-line, and when armoured sides and protecting decks were introduced sails were dispensed with altogether. The trunk engine remained a favourite model until it became impossible to keep the trunks in a steam-tight condition owing to the adoption of high-pressure steam. The use of a geared wheel for multiplying the number of the revolutions of the screw shaft was continued until the crank shaft could be connected direct to the screw shafting. Up to about 1860 horizontal engines were the rule in warships, and though not particularly economical in the matter of fuel, they were a great improvement on the type which had been so useful for paddle engines. Surface condensation became general about 1860, and made possible the introduction of compound engines and cylindrical boilers. The old flat-sided boilers were retained when surface condensation was first introduced, but additional stays were added to enable them to stand the increased steam pressure which had by now advanced from 4 lb. to the square inch, at which it was deemed effective in the early days of the steamship, to 30 to 35 lb. to the square inch.