In this form of engine the steam chamber is partly spherical, and the end-covers are cone-shaped, while the chamber contains a piston or circular disc fitted with a central boss that fits into spherical seats made in the covers, and a projecting arm placed at right angles to the disc engages with a crank arm on the screw shaft. A fixed radial partition intersecting the disc divides the chamber into four cells, to which steam is admitted by a slide valve. In 1849 H.M.S. _Minx_ was equipped with one of these engines having a disc of 27 inches diameter, in addition to the high-pressure engine, and coupled to the propeller shaft in such a manner that it was not necessary to disconnect the horizontal engines. With the disc engine the vessel attained a speed 11 per cent. higher than without. Improvements in other engines, however, rendered inevitable the relegation of the disc engine to the list of superseded contrivances. In 1838 Mr. John Penn’s oscillating engines with tubular boilers were fitted in some of the boats running above London Bridge, and attracted the attention of the Admiralty. The Admiralty yacht _Black Eagle_ was turned over to him and he installed, instead of her former engines, oscillating engines of double their power, with tubular flue boilers, the change entailing no addition to the weight or engine space. The advantages of this installation were so great that many other vessels were similarly treated, among them being the royal yacht _Victoria and Albert_. His trunk engine, designed for the propulsion of warships carrying a screw, and capable of being placed below the water-line so far as to be out of reach of hostile shot, achieved an even greater success, and in 1847 Mr. Penn was instructed to place engines of this type in H.M.S. _Arrogant_ and H.M.S. _Encounter_. These were so satisfactory that orders for engines were received for vessels ranging from a small gunboat, to be fitted with engines of 20 horse-power, to vessels like the _Sultan_, with engines of 8629 horse-power, and _Neptune_ (ex _Independencia_), with 8800 indicated horse-power. Up to the time of his death his firm fitted 735 vessels with engines having an aggregate actual power of more than 500,000 horses. Among them were the _Orlando_, _Howe_, _Bellerophon_, _Inconstant_, _Northampton_, _Ajax_, _Agamemnon_, _Hercules_, _Sultan_, _Warrior_, _Black Prince_, _Achilles_, _Minotaur_, and _Northumberland_.