IBB LOCOMOTIVE THE SCREW-PROPELLER.] APPLIED MECHANICS. 



881 



carriages ; and accordingly it was proposed to apply 

 level's to act as legs and feet propelling the carriage, or 

 to lay down a continuous rack, in y.'hich a toothed wheel 

 driven by the engine might work, or an endless chain 

 to be wound round a revolving barrel. It was not until 

 the actual trial was made, that the simple expedient of 

 causing a pair of wheels to revolve on the rails, was 

 *dopted without reserve. 



The construction of the boiler and engines of a loco- 

 motive has been already referred to (page 866). The 

 whole power is applied to turn the crank-shaft, on which 

 are fixed the driving-wheels. For each revolution of 

 the engine, therefore, there is one revolution of the 

 driving-wheels. Sometimes these are made 6 to 7 feet 

 in diameter, and have therefore a circumference of 20 

 feet. If we suppose the engines to make 200 revolutions 

 per minute, or 200 X 60=12,000 per hour, as the whole 

 circumference of the driving-wheel is brought to bear 

 on the rail during each revolution, if there be no 

 slip, the locomotive must advance 20 feet for each re- 

 volutionthat is to say, 12,000 X 20=240,000 feet, 

 above 45J miles per hour. When the rails are damp 

 and slippery, the wheels occasionally slip round without 

 taking sufficient hold of them to propel the train. In 

 such cases, a little sand strewn on the rails generally 

 ! causes sufficient friction to commence the movement of 

 the train ; and its momentum once established, the loco- 

 motive has only to keep it up against the resistance of 

 the air, and the friction of the running wheels. When 

 the locomotive is applied to the propulsion of heavy 

 goods-trains, especially up inclinations, it is necessary 

 to obtain more friction than could be derived from one 

 pair of driving-wheels. For this purpose the locomotive 

 is fitted with one, and sometimes two additional pairs of 

 driving-wheels, of the same size as those worked by the 

 engine. These wheels are coupled by connecting-rods 

 working on pins projecting from their arms, so that they 

 all revolve simultaneously, and thus present double or 

 treble the amount of rubbing surface on the rails. The 

 locomotive has attached to it a tender, which is an iron 

 tank mounted on wheels, divided into two compart- 

 ments, one containing fuel and the other water for the 

 supply of the engine. The water-tank is connected by a 

 flexible tube to the feed-pump of the locomotive, by 

 which the pump draws its supply, and forces it into the 

 Ixrtler. In order to save fuel, the water in the tank is 

 heated by steam blown through it from the boiler, while 

 the traiu is stopping, when the steam is blowing off to 

 waste by the safety-valve. Occasionally the water-tank 

 is fixed on the top of the boiler, and the tender is dis- 

 pensed with. Tlie engine-driver and stoker stand on a 

 stage at the fire-box end of the boiler, and have under 

 their eye the water and steam-gauges. They have 

 conveniently arranged levers for working the steam- 

 valve, so as to permit more or less steam to enter the 

 cylinder ; for moving the link-motion of the slides, 

 so as to reverse the motion of the engines ; for 

 adjusting the pressure on the safety-valve, the 

 supply of feed-water, and the break on the wheels 

 when it is necessary to stop or move more slowly. 

 They have convenient means of oiling all the working f , 

 parts, and of opening pet-cocks in the cylinders / 

 to permit the issue of water, and the cock for / 

 sounding the steam- whistle as a signal of their > 

 approach. Altogether, the locomotive is perhaps \ 

 the most perfect apparatus that has been designed \ 

 by engineers ; and the materials and workmanship 

 applied in its construction are necessarily of the 

 best kind, to withstand the constantly reiterated 

 shocks of the movement, and to convey power so 

 great through parts so light and apparently so complex. 

 IV. PROPULSION or VESSELS. Although numerous 

 modes of applying steam to the propulsion of vessels 

 have been proposed, only two have met with general 

 adoption viz., the paddle-wheel and the screw-pro- 

 peller. The action of paddle-wheel engines is pre- 

 cisely similar to that of locomotives : the paddles in the 

 one case occupying the place of the driving-wheels in 

 the other, and having float-boards successively immersed 



VOL. I. 



in the water, and withdrawn from it as the wheels re- 

 volve. The paddle of a steam-vessel is, indeed, an un- 

 dershot water-wheel reversed in its action ; that is to 

 say, instead of the wheel being put in motion by the 

 current of water, the wheel put in motion by the steam- 

 engine causes a current of water by its revolution ; and 

 the reaction of the water propelled by it backwards, 

 forces the wheel forwards, and with it the vessel to which 

 it is attached. As the float-boards enter and leave the 

 water, they act on it obliquely, tending in some measure 

 to press it down in front and raise it behind. In sea- 

 going vessels, where the wheels often act on the undu- 

 lating surface of the water, this obliquity of action 

 becomes a very considerable resistance, and tends to 

 retard the engines and give them severe shocks. To 

 obviate this defect, paddles are frequently made in such 

 a manner that while passing through the water the floats 

 hang nearly vertical Such a wheel is called a featliering 

 paddle, because the float-boards feather, or enter and 

 leave the water edgeways, like the oar in the hands of a 

 practised rower. The power required for a steam-vessel 

 depends upon its form and tonnage, and the speed at 

 which it is moved. 



Steam -vessels are generally made very long in pro- 

 portion to their breadth, and finely wedge-shaped at each 

 end, so as to cut through the water with as little re- 

 sistance as possible. In vessels of similar form and pro- 

 portions, the power required to produce a given speed is 

 nearly as the tonnage. But a very slight increase of 

 speed demands a very considerable augmentation of 

 power, as may be thus estimated : To double the speed 

 of a vessel it is necessary to push aside double the 

 quantity of water in a given time, and to impel this 

 water with double the velocity, and therefore to encounter 

 four times the resistance ; and, as the speed of the 

 engines must be at least doubled, the power expended 

 in a given time must be at least eight times. Generally 

 the power may be taken as the cube of the speed, or 

 rather more. If, for example, a pair of engines working 

 to 10J horse-power, propelled a vessel at the rate of 8 

 knots (nautical miles) per hour, we should have to work 

 the engines up to 200 horse-power to attain a speed of 

 10 knots per hour ; because while the cube of 8, or 8 X 

 8 x 8 = 512, the cube of 10 is 1,000, nearly double of 

 512, and therefore the power iu the one case must be 

 double of that in the other. The same law applies in 

 the case of vessels propelled by a screw. 



The principle upon which the screw acts as a propeller, 

 may be best understood by considering the action of a 

 windmill reversed. ' The screw generally consists of two 

 inclined blades (Fig. 211) projecting from an axis 

 mounted in bearings, in what is called the dead-wood of 



Fig. 211, 



a vessel, or the part of the stern immediately before the 

 rudder. The screw-shaft works through a water-tight 

 tube fixed in the dead-wood, and is put in motion by 

 the steam-engines in the body of the vessel. The whole 

 screw is immersed under water, and the blades are of 

 such size that, looking endways on the shaft, they appear 

 to occupy each about one-sixth of the circle which cir- 

 cumscribes them. 



The pitch of the screw depends on the obliquity of the 



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