MARITIME CONVEYANCE. 



used over and over again, the small losses that 

 occur being made up by a small addition of salt 

 water when necessary. The piston-rods and con- 

 necting-rods are lettered N and O respectively, 

 and the crank-shaft, P. Q is a cast-iron bed- 

 plate, to which the upper parts of the engine are 

 securely bolted ; and R, R, are wrought-iron 

 columns supporting the front of the cylinders. 



Many details are necessarily omitted from the 

 figures, but enough are shewn to give an idea of 

 the working of the engine. It was a favourite 

 type long before the compound principle came 

 to be acknowledged ; and the only external 

 difference of any importance was, that the two 

 cylinders were the same size. This is not the 

 place to enter into arguments of a purely engi- 

 neering nature for or against compound engines. 

 We need only say that, without exception, they 

 are being adopted by all the great steam-shipping 

 companies ; and that a careful examination of the 

 work done by marine engines all over the kingdom 

 during the last half-dozen years, would shew 

 that they are superseding ordinary engines (for 

 marine purposes) as rapidly as the screw has 

 superseded the paddle. They are universally 

 found to effect a very great saving in fuel ; but, 

 besides this, owing to the more advantageous 

 distribution of the steam, the ratio of the dynamo- 

 metrical or actual horse-power given out by them 

 to the indicated horse-power is considerably 

 greater than in engines working with anything 

 like the same economy, but uncompounded. The 

 effect of this is, that a certain indicated horse- 

 power given out by a compound engine propels a 

 given ship faster than the same indicated horse- 

 power from an ordinary engine.* The results of 

 numerous experiments seem quite conclusive on 

 this point. 



The engines in a steamer are generally placed 

 as far aft as possible, the width of the vessel and 

 the height of the bed-plate determining their 

 position. The shaft is often made to lie at a 

 small angle to the keel, for the sake of getting 

 more room for the engines ; although some engi- 

 neers consider it an advantage on other grounds 

 as well to have the shaft lying a little downwards 

 towards the screw. In engines of more than 50 or 

 60 nominal horse-power, there are generally two 

 or more boilers. These are always placed forward 

 of the engines, and are now generally made 

 cylindrical, in order the better to resist the enor- 

 mous pressure of the confined steam, which is 

 often as great as 60 or 70 Ibs. per square inch, or 

 more than 4 tons per square foot. An engraving 

 and description of a marine boiler will be found in 

 the article STEAM-ENGINE. The fuel used in the 

 boilers is always coal of a superior quality, and 

 the great object of the engineer is to construct 

 boilers and engines which shall perform a maxi- 

 mum of work with a minimum of fuel. First-rate 

 steamers with such engines as have been above 

 described, will not use more than 2 Ibs. of coal per 

 indicated horse-power per hour. Even at this low 

 rate of consumption, a steamer with engines of 

 500 horse-power nominal (or about 2000 horse- 

 power indicated) would use over 400 tons of coal 

 on one voyage across the Atlantic. 



Some London engineers, Messrs J. and W. 

 Dudgeon, have done more in making twin-screw 



* This would not be the case, or, at least, not at all to such an 

 extent, were it possible to fit marine engines with large fly-wheels. 



steamers and engines than any other firm. The 

 style of engines generally adopted by them of late 

 years is very neat, and economical of space. They 

 are called diagonal engines, and are in the form of 

 the letter A, the cylinders being at the top, and 

 sloping down each to their own screw-shafts, which 

 are at the bottom of the legs. The condenser and 

 pumps are arranged to fill up the triangular space 

 in the middle. 



It has often been proposed to propel vessels by 

 jets of water forced through nozzles at their sides, 

 the reaction of the sea against these jets driving 

 the vessel forward. With this system of hydraulic 

 propulsion, the name of Mr Ruthven is associated. 

 In 1866 he succeeded in getting government to try 

 his plan in the Waterwitch, a sister gun-boat to 

 the twin-screw vessels Viper and Vixen. The 

 engines were 160 horse-power nominal, and had 

 three cylinders driving a large turbine, or centrif- 

 ugal pump, which drew water from the bottom of 

 the ship, and discharged it through the side-nozzles. 

 This vessel was fairly successful, but not so much 

 so as to make the Admiralty see their way to the 

 further adoption of the hydraulic principle. 1 1 pos- 

 sesses some advantages notably, that the engines 

 would always turn in the same direction, and the 

 reversing of the ship's motion would be effected by 

 the altering of the direction of the jets by the cap- 

 tain on deck ; that the turbine could be so made as 

 to pump from the interior of the ship if necessary, 

 and in this way a very considerable leak might be 

 rendered harmless ; and that no moving propeller 

 whatever existed which could be damaged or 

 fouled in any way. The chief defect seemed to lie 

 in the design of the so-called turbine, and it is prob- 

 able that when this has been more fully investi- 

 gated, more will be heard of hydraulic propulsion. 



NAVIGATION. 



Navigation is the art of conducting vessels at 

 sea in the direction in which they are designed to 

 proceed. It will not be out of place here to say a 

 few words regarding the management of a ship 

 while on her voyage, and the instruments used by 

 the officers to find out their position, speed, and 

 direction, although we cannot do much more than 

 mention them. 



Tides, currents, and winds are the three natural 

 agencies which advance a sailing-vessel on its 

 course ; the last is, of course, the most important, 

 and the skill of the mariner shews itself in the 

 degree to which he succeeds in rendering almost 

 every breath of wind, from whatever quarter, usc- 

 : ul in propelling the ship in the desired direction. 

 The most favourable winds are those which blow 

 on the quarter, or slantingly on the ship's course. 

 The reason for this is, that when the wind blows 

 directly astern, the sails furthest aft intercept it, 

 and prevent its having any action on those further 

 forward, so that only a few sails arc actually of 

 use in propelling the ship. When the wind is on 

 :he quarter, however, all the sails, when properly 

 braced (turned round), can catch the breeze at the 

 same time. The variety in rigging of different 

 vessels causes great difference in their sailing 

 power ; some will sail much closer to the wind 

 Lhat is, much more nearly towards the direction 

 from which it is blowing than others. When the 



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