422 Machinery at the Exhibition. 



species of steam engine now in nse that was not at that time well 

 known and skilfully constructed ; but the extent to which the 

 various kinds are used has undergone some modification, since 

 the oscillating-engine is less and the trunk-engine more com- 

 monly employed. Certain important alterations also have been 

 made in the form and in the arrangement of marine-engines. 

 These are due to the screw-propeller having in a great measure 

 superseded the paddle-wheel. At the period of the last Exhi- 

 bition, almost every steam vessel was furnished with paddles ; 

 but experiments had recently been made, which rendered it 

 probable that the screw would be applied to vessels of Avar, 

 although no idea was then entertained that it possessed advan- 

 tages which have caused it since to be almost universally 

 adopted in the navy. The change which has thus taken place 

 has rendered a considerable modification of the marine-engine 

 necessary ; it must have a higher velocity, and the crank-axle 

 must occupy a very different position from that which was 

 formerly assigned to it. There is also a great and general 

 endeavour to diminish the space occupied in steam-vessels by 

 the engines; hence a great compactness has been given to 

 them, and a great condensation of their parts has been effected, 

 and this has been accompanied by an augmentation of their 

 strength and solidity. This is effected in various ways ; the 

 beam, in every form, has been nearly discarded, and direct- 

 action engines are almost always used in steamers. When the 

 oscillating engine is employed, there is no connecting-rod; 

 when the trunk-engine, there is no piston rod ; and the use of 

 a double piston-rod, causes the piston and connecting-rods to 

 occupy only the space usually required by one of them. More- 

 over, the position and form given to the condenser and pumps, 

 tends more or less to the economization of space. When the 

 screw was first applied, the required velocity was obtained by 

 gearing, or some similar contrivance ; but these methods have 

 been abandoned, and the screw-shaft is driven to its full speed 

 by the engines themselves. In 1851, the largest steam-engines 

 exhibited were of but six hundred horse power ; those in the 

 present Exhibition include engines of eight hundred horse- 

 power, and portions of others of twelve hundred and fifty, and 

 even of thirteen hundred and fifty. Nothing can exceed these 

 admirable specimens, in the arrangement of their details, and 

 the excellence of their finish. Stationary engines also have 

 been modified to some extent, chiefly by a more general use, 

 and a more simple application of the condenser and of the 

 principle of expansion. The latter was well understood long 

 since, bat it is now being generally applied in the best manner, 

 and in most cases with a power of varying its extent at 

 pleasure. 



