STEAM NAVIGATION. 



a number of small cylinders, ranged often in a horizontal position 

 on either side of the screw-shaft, allow of the play of all the 

 reciprocating parts within a small height, so as to keep the whole 

 below the water-line. 



83. Another expedient for the protection of the machinery from 

 shot, is to place the coal-boxes on each side of it, and between it 

 and the timbers of the vessel, so that before a shot could reach it, 

 the fuel must be thoroughly penetrated. 



84. The efficiency of a marine, like that of a land engine, depends 

 on the exact regulation of the slides by which the admission and 

 escape of the steam to and from the cylinder is governed. In all 

 cases the steam should be admitted at either end of the cylinder a 

 little before the arrival of the piston there, and at the same 

 moment the escape to the condenser should be stopped. By this 

 means the piston, on arriving at the end of the stroke, is received 

 by the steam just admitted mixed with a small portion of uncon- 

 densed steam and air, whose escape to the condenser has been 

 intercepted. These form a sort of air-cushion, against which the 

 stroke of the piston is broken, an effect which is called by the 

 practical men, not inappropriately, cushioning the piston. When 

 the steam is worked expansively, the slides must be capable of 

 such regulation as to shut it oft' at any required fraction of the 

 entire stroke, and when not so worked, it ought at all events to 

 be shut off before the stroke is quite completed, so as to relieve 

 the piston from its action a little before the termination of the 

 stroke. 



It is easy to conceive that, to accomplish all these points, the 

 slides require the nicest imaginable adjustment ; and the openings 

 for the admission and escape of steam, the most exact regulation 

 both as to magnitude and position. 



85. It will be evident on comparing the pitch of the ordinary 

 screw with the progressive rate at which the vessel moves through 

 the water, that, to produce the necessary speed, a much greater 

 velocity of rotation must be imparted to the screw, than is con- 

 sistent with the ordinary rate at which steam-engines work. It 

 has been already shown that this great velocity of rotation has 

 been obtained either by the interposition of gearing so adapted as 

 to augment the velocity, or by assimilating the engine in its form 

 and structure to a locomotive. 



86. An example of a marine-engine, by which the necessary velocity is 

 imparted to the screw-shaft, by means of intermediate gearing, is pre- 

 sented in the case of the screw-engine constructed by Messrs. Penn and 

 Son, for the " Great Britain" steam-ship. The engines which are represented 

 in fig. 26, are constructed on the oscillating principle, and are almost 

 identical with the paddle-wheel engines, built by the same firm for 

 the "Sphinx." 



168 



