OSCILLATING ENGINES. 



sometimes three, sometimes four, and sometimes, though very 

 exceptionally, only one. 



The position of the cylinders is subject to great variation. 

 They are placed with their axes sometimes vertical, sometimes 

 horizontal, and sometimes oblique. 



51. The proportion of the diameter to the stroke is subject to 

 like variation. The general tendency has been to increase the 

 relative magnitude of the diameter, which in recently built 

 engines is sometimes more than twice the stroke, and rarely less 

 than two-thirds of it. Thus in the engines of the " Niger," con- 

 structed by Messrs. Maudslay and Field, the cylinders have 48 

 inches diameter, with only 22 inches stroke ; and in the " Simoom," 

 by Boulton and Watt, they have 44 inches diameter, with 30 

 inches stroke. 



The object of shortening the stroke is to diminish the momentum 

 of the piston, of which the motion requires to be so frequently 

 reversed. 



52. In engines constructed on the oscillating principle, the top 

 of the piston-rod is coupled with the crank, and the piston-rod 

 moves backward and forward in the direction of the axis of the 

 cylinder, while its extremity revolves in a circle with the crank. 

 It is therefore necessary that the cylinder should oscillate from 

 side to side, to accommodate the motion of the piston-rod to that 

 of the crank. For this purpose the cylinder is provided on each 

 side with a short hollow pivot or trunnion, on which it swings ; 

 and through one of these trunnions the steam enters the cylinder 

 from the boiler, while it escapes through the other to the condenser. 

 The alternate admission and escape of steam on the one side and 

 the other of the piston, is regulated by a valve attached to the 

 cylinder and swinging with it. In the larger class of engines, 

 however, two valves are usually employed for this purpose, and 

 are so arranged as to balance one another. 



Oscillating engines are usually placed immediately under the 

 cranks, and occupy no greater length in the vessel than the 

 diameter of the cylinder. On the shaft which connects the 

 engine, called the intermediate shaft, a crank is forged which 

 in its revolutions gives motion to the piston of the air-pump. 



53. The arrangements generally employed at present in the most 

 improved vessels propelled by oscillating engines, will be understood by 

 reference to fig. 10, which represents the transverse section of the steam- 

 yacht "Peterhoff," constructed for the Emperor of Russia, by Messrs. Rennie, 

 and fig.' 11, which is a side view of the engines of the same vessel. These 

 figures are copied with the permission of the publishers and the authors, 

 from the article on the steam-engine, in the last edition of Brande's 

 "Dictionary of Science." A, A are the cylinders ; B, B are the piston-rods, 

 which are connected immediately with the cranks c, c ; D is a crank on 



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