MARINE ENGINES. 



some respects from that of a land engine. The limitation of space, 

 which is unavoidable in a vessel, renders greater compactness 

 necessary. The paddle-shaft on which the cranks to be driven by 

 the engine are constructed being very little below the deck of the 

 vessel, the beam, if there be one, and connecting rod could not be 

 placed, in the position in which they usually are in land engines, 

 without carrying the machinery to a considerable elevation above 

 the deck. This is done in the steam-boat engines used on the 

 American rivers ; but it would be inadmissible in steam-boats in 

 general, and more especially in sea-going steamers. The connect- 

 ing rods, therefore, instead of being presented downwards towards 

 the cranks which they drive, must, in steam- vessels, be presented 

 upwards, and the impelling force be received from below. If, 

 under these circumstances, the beam were in the usual position 

 above the cylinder and piston-rod, it must necessarily be placed 

 between the engine and the paddle-shaft. This would require a 

 depth for the machinery which would be incompatible with the 

 magnitude of the vessel. The beam, therefore, of marine engines, 

 instead of being above the cylinder and piston, is placed below 

 them. To the top of the piston rods, cross-pieces are attached, 

 of greater length than the diameter of the cylinders, so that their 

 extremities shall project beyond the cylinders. To the ends of 

 these cross-pieces are attached by joints the rods of a parallel 

 motion: these rods are carried downwards, and are connected 

 with the ends of two beams below the cylinder, and placed on 

 either side of it. The opposite ends of these beams are con- 

 nected by another cross-piece, to which is attached a connecting 

 rod, which is continued upwards to the crank-pin, to which it 

 is attached, and which it drives. Thus the beam, parallel motion, 

 and connecting rod of a marine engine, are similar to those of a 

 land engine, only that they are turned upside down ; and in con- 

 sequence of the impossibility of placing the beam directly over the 

 piston rod, two beams and two systems of parallel motion are pro- 

 vided, one on each side of the engine, acted upon by, and acting 

 on the piston rod and crank by cross-pieces.* 



The proportion of the cylinders differs from that usually ob- 

 served in land engines for like reasons. The length of the 

 cylinder of land engines is generally greater than its diameter, 

 in the proportion of about two to one. The cylinders of marine 

 engines are, however, commonly constructed with a diameter 

 greater than their length. In proportion, therefore, to their 

 power their stroke is shorter, which infers a corresponding short- 



* We must assume that the reader of the present Tract has already 

 rendered himself familiar with the several Tracts on Steam and the Steam 

 Engine, already published in the Museum. 



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