WAR STEAMERS. 



before a committee of the House of Commons in 1850, and the 

 report founded thereupon, it appeared that commercial steamers 

 in general are capable of war service, with no other previous 

 alteration or preparation than such as are easily practicable and 

 expeditiously executed. It was shown that all steamers of 400 

 tons and upwards would be capable, with some additional strength- 

 ening, to carry such pivot guns as are used in war-steamers, and 

 that there are few mercantile steamers of any size, which might 

 not carry an armament such as would render them useful in case 

 of an emergency. 



22. The principle on which the steam-engine is applied to the 

 propulsion of ships is the same as that by which oars act in pro- 

 pelling boats. In both cases the propelling instruments having a 

 point or points of reaction on the vessel, are made to drive a mass 

 of water backwards, and the moving force, or momentum, thus 

 imparted to the water from stem to stern, is necessarily attended 

 with a reaction from stern to stem, which, taking effect on the 

 vessel, gives it a corresponding progressive motion. 



By the well-known mechanical principle of the composition and 

 resolution of force, it can be demonstrated that whatever force 

 may be imparted to the water by the propeller, such force can be 

 resolved into two elements, one of which is parallel, and the 

 other in a plane at right angles to the keel. The former 

 alone can have a propelling effect, and since the latter is wholly 

 ineffective, the propeller should always be so constructed that its 

 whole force, or at least the chief part of it, shall be employed in 

 driving the water in a direction parallel to the keel from stem to 

 stern. 



23. The mechanical expedients by which the power of steam is 

 rendered available for the propulsion of vessels are very various, 

 both as regards the form of the engine which acts upon the pro- 

 peller, and the form of the propeller itself. 



In all cases hitherto reduced to practice, the propeller is awheel 

 fixed upon a horizontal shaft, to which the engine imparts a 

 motion of continued rotation. The wheel is so constructed that 

 when it revolves it imparts to a volume of water, more or less 

 considerable, a motion either directly backwards, or one whose 

 principal component has that direction. The greater the pro- 

 portion which this principal component has to the entire force 

 exercised by the propeller, the more effective it will be. 



24. The propellers hitherto practically applied in steam-naviga- 

 tion are of two kinds, called, paddle-wheels and screws. 



The shaft of the paddle-wheels is fixed horizontally across the 

 vessel, and consequently at right angles to the direction of the 

 keel. 



