CHAP. VIII.] 



STEAM NA VIGA TION. 



459 



the blades of its screw felt the sensation of a current of air produced 

 by the motion -of its enormous whirls. This engine is also remarkable 

 as a type with special qualities, on which we will say a few words in 

 conclusion. It is an expansion engine on WoolfFs system, with this 

 peculiar arrangement, that it comprises three equal cylinders of the 

 same diameter and the same height. The steam is at first introduced 

 into one cylinder only, the middle one ; after having worked at full 

 pressure in this, it then enters the two lateral cylinders, where it 

 expands, and passes thence into two separate condensers. On 

 leaving the boiler, the steam circulates in a drying apparatus, and 



Fie;. 312. Combined engines of tlie Frledland. 



then bifurcates into the steam jackets of the extreme cylinders. 

 For equal power and equal weight of engines a remarkable economy 

 of fuel is obtained with this system, as compared with the double 

 cylinder system. The knees or cranks of the driving shaft, which 

 receive the heads of the connecting-rods are arranged at right angles 

 in the corresponding knees of the two outside pistons, and in the 

 bisector of that angle produced in the case of the middle knee, the 

 advantage results, that all the movable pieces keep very nearly the 

 same position of equilibrium about the axis of the shaft, whatsoever 

 may be the position of the ship as determined by rolling. 



