EXPANSIVE ACTION. 



much practical experience in steam-vessels, that furnaces six feet 

 in depth from front to back should not be less than three feet 

 in width to afford means of firing with as little injury to the side 

 plates as possible, and of keeping the fires in the condition 

 necessary for the production of the greatest effect. The tops of 

 the furnaces scarcely ever decay, and are seldom subject to an 

 alteration of figure, unless the level of the water be allowed to 

 fall below them. 



47. The method by which the greatest quantity of practical 

 effect can be obtained from a given quantity of fuel must, how- 

 ever, mainly depend on the extended application of the expansive 

 principle. This has been the means by which an extraordinary 

 amount of duty has been obtained from the Cornish engines. The 

 difficulty of the application of this principle in marine engines, has 

 arisen from the objections entertained in Europe to the use of 

 steam of high-pressure, under the circumstances in which the 

 engine must be worked at sea. To apply the expansive principle, 

 it is necessary that the moving power at the commencement of the 

 stroke shall considerably exceed the resistance, its force being 

 gradually attenuated till the completion of the stroke, when it 

 will at length become less than the resistance. This condition 

 may, however, be attained with steam of limited pressure, if 

 the engine be constructed with a sufficient quantity of piston 

 surface. 



48. This method of rendering the expansive principle available at sea, 

 and compatible with low-pressure steam, was projected and executed by 

 Messrs. Maudslay and Field. Their improvement consists in adapting 

 two steam cylinders in one engine, in such a manner that the steam shall 

 act simultaneously on both pistons, causing them to ascend and descend 

 together. The piston-rods are both attached to the same horizontal cross- 

 head, whereby their combined action is applied to one crank by means of a 

 connecting-rod placed between the pistons. 



A section of such an engine (which has been called the Siamese engine), 

 made by a plane passing through the two piston-rods p p' and cylinders, is 

 represented in fig. 9. The piston-rods are attached to a cross-head c, 

 which ascends and descends with them. This cross-head drives upwards 

 and downwards an axle D, to which the lower end of the connecting-rod 

 E is attached. The other end of the connecting-rod drives the crank - 

 pin P, and imparts revolution to the paddle-shaft a. A rod H conveys 

 motion by means of a beam I to the rod K of the air-pump L. 



Engines constructed on this principle were applied in several steamers, 

 and amongst others in her Majesty's steam-frigate "Retribution." 



49. Tyithin the last ten or fifteen years, and especially 

 since the more general adoption of the screw-propeller, the 

 marine engine has been greatly simplified in its mechanical 



Its bulk has thus been diminished, as well as 

 L 2 H7 



