CHAP. VIIL] STEAM: NAVIGATION. 457 



made fast to it by means of a cross-head brought down by side pieces. 

 At the other end of the beam there is a connecting-rod to the crank. 

 In this form, the rod connecting the crank and the end of the beam 

 may be longer ; the power is thus delivered to the crank more equally, 

 and the weights of the moving parts are balanced, so that a slight 

 pressure of steam is able to drive the engine both ahead and astern, 

 the parts being in equilibrium. But there is an objection to this kind 

 of engine, the parts are heavy and not compact, these qualities 

 being most detrimental in a war-steamer. 



Oscillating Engine. This engine has derived most of its elegance 

 and perfection from Mr. Penn. The name of the engine is derived 

 from the fact that the cylinders oscillate upon hollow axes or 

 "' trunnions," through which the steam is admitted and withdrawn 

 from the valves ; the piston-rod connects itself to the crank without 

 the use of any extra gearing ; in fact this is one of the most direct- 

 acting engines known, and is used in the largest ocean as well as the 

 smallest river steamers, in the latter of which it was first tried by 

 Maudsley, 



Annular Engine. In this variety of engine the cylinder is made in 

 the form of a ring with a central cylinder. There are. two piston-rods 

 made fast to a cross-head in the form of a "|~, the tail of which works 

 in the central cylinder between sliding faces, the connecting-rod 

 being fastened at the end of the tail, which transmits the power to 

 the crank. This kind of engine allows a long connecting-rod and 

 therefore the paddle-shaft high. 



Fig. 311 represents a side-lever engine. The beam oscillates below 

 the cylinder and piston an arrangement rendered necessary by the 

 situation of the driving shaft, or the axis of the paddles, which 

 necessarily occupies an elevated position in paddle-ships. The con- 

 necting-rods are joined immediately to the shaft, which is bent at two 

 points so as to form two cranks at a right angle, each working with 

 one cylinder. The cylinders are vertical. When the same type of 

 machines is applied to the screw the cylinders are placed horizontally 

 across the ship. Sometimes direct horizontal machines with two 

 cylinders are preferred, and then the connecting-rods work on a crank 

 on the shaft of the screw itself. 



The cylinders of marine engines are often of colossal dimensions. 

 Those of the Great Eastern are 7 feet in diameter, with a 4 foot 



