454 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



20 per cent, of the propelling power, without giving any useful effect, 

 as the blade is then nearly in a line with the shaft. 



Several experiments have been made lately with twin screws, 

 arranged one on each side of the keel, instead of a single one in the 

 axis of the vessel. Great facility is afforded by this arrangement for 

 turning and steering with or without a rudder, but so far as speed is 

 concerned, no advantage is gained. 



IV. MAKINE BOILERS AND ENGINES. 



We are now acquainted with the propellers for steamships, and we 

 must next inquire how steam, the only motive force sufficiently 

 powerful to fall back upon as a substitute for the inconstant and 

 often contrary force of the wind, gives to the wheels or the screw 

 their rotatory motion. 



Is the steam-engine, such as we have described it, modified in any 

 essential particulars when it becomes a marine engine ? 



No. In reality, not only is the principle identical, but the chief 

 parts the generator, the driving and transmitting machinery remain 

 the same. They have only, as we shall see, to submit to the particular 

 necessities of being placed in a ship. 



At first, low-pressure engines with condensation that is to say, 

 Watt's beam engines which were the only ones elsewhere employed 

 in industry formed the type of navigating engines, whether on 

 rivers, lakes, or seas ; and paddle-boats still use them with advantage. 

 Their motion is comparatively slow, but as is well known, this slow- 

 ness is largely compensated for by the regularity of their working. 

 They are unwieldy arid cumbersome certainly, but all their parts are 

 easily accessible for inspection, maintenance, and, when needed, for 

 repairs. These engines were adopted in the navies of England and 

 France before the invention of the screw had changed the conditions 

 of the problem. For working the screw, these engines give too slow 

 a motion of rotation^-which would no doubt be easy to multiply by 

 cog-wheels, but at the expense of the effective force of the engines, 

 in other words, of their available work. 



Condensation is generally adopted not only where it is necessary, 

 that is to say in low-pressure engines ; bub also in marine engines 



