STEAM NAVIGATION. 



the cost of construction; and the profitable tonnage of the 

 vessel has been increased in a corresponding proportion. The 

 beam and its appendages have been very generally laid aside, 

 and the piston-rods have been more directly connected with 

 the cranks. 



In some cases the piston-rods are kept in their direction by 

 guides, and their rectilinear motion is accommodated to the 



Fig. 9. 



rotation of the cranks by connecting-rods, which consequently 

 have an oscillation between the extreme points of the play of the 

 cranks. 



In other cases the cylinders themselves receive this oscillation. 

 In such cases the connecting-rods are dispensed with, and the 

 ends of the piston-rods are immediately jointed to the cranks. 

 The oscillation of the piston causes the motion of the valves neces- 

 sary for the alternate admission and escape of the steam on the 

 one and the other side of the piston. 



50. The number of cylinders varies, being generally two, but 

 148 



