CHAMBERS'S INFORMATION FOR THE PEOPLE. 



draught steamers, where a propeller of the requi- 

 site size would only have been partially immersed, 

 and where, for some reasons, paddle-wheels were 

 inadmissible or undesirable, vessels have been 

 fitted with two screws, each, of course, being pro- 

 portionately less in diameter than a single one 

 would have been to exert the same power. In 

 these cases one propeller is placed on each side 

 of the ship, under the quarter that is, very nearly 

 abreast of the place for the single screw. Steamers 

 fitted in this way are called double or twin-screw 

 vessels. They are always fitted with two entirely 

 separate sets of engines, one to each screw-shaft, 

 and they thus possess the advantage very useful 

 to war-steamers, and in some cases of impending 

 collision, useful to any steamer that by reversing 

 one of the engines while the other is going ahead, 

 the vessel can be turned round almost as if pivoted 

 on her own centre. The additional expense and 

 trouble, however, of having engines, shafting, and 

 propellers all in duplicate, prevent the twin-screw 

 system being adopted under any circumstances 

 except those named above. 



Screw-propeller shafts run very much more 

 quickly than the shafts of paddle-wheels. At 

 first, however, it was attempted to use in screw- 

 steamers engines the same as those used for driving 

 paddle-wheels, making up for the difference of 



speed by using toothed gearing between the screw 

 and engine shafts. Such useless complication was 

 soon found to be wasteful, and engines were made 

 to work the screw-shaft direct, just as they had 

 previously worked the paddle-shaft direct ; so that 

 each revolution of the shaft should correspond to 

 a double stroke of the piston. For an exposi- 

 tion of the principles and construction of the 

 steam-engine, we must refer to No. 27 ; but 

 in this place we must give a short description 

 of the type of engine now almost universally used 

 although modified in various ways by various 

 makers in merchant steamers of moderate size. 

 Figs. 9 and 10 shew a direct -acting, surface- 

 condensing, compound screw-engine. It is called 

 direct acting, because the motion of the piston 

 is transmitted to the crank direct, without 

 the intervention of anything but a connecting 

 rod ; surface condensing, because the steam is 

 condensed on the surface of a number of small 

 tubes, kept cool by a current of water inside them ; 

 and compound, because the steam is admitted 

 direct from the boiler to only one cylinder, and to 

 the other and larger cylinder only after it has done 

 its work in the first. The objects and advantages 

 of surface-condensation and the compound prin- 

 ciple have been fully entered into in the article 

 above referred to, with which we must presume 



Fig. 9. 



the reader of this to be familiar. Fig. 9 is a 

 vertical section of the engine in the plane of the 

 screw-shaft ; and fig. 10 an end-view of the same 

 engine, looking aft. 



A is the main steam-pipe from the boilers ; B, 

 the smaller or high-pressure cylinder, with its 

 piston, C, in which the steam commences its work. 

 D is a slide-valve, regulating the distribution of 

 the steam to B. When the steam has done its 

 work in B, it is allowed by the valve to pass into 

 a receiver, E, and thence another slide-valve, F, 

 admits it to the low-pressure cylinder, G. It is 

 here expanded until its pressure is much less than 



458 



Fig. 10. 



that of the atmosphere, and then allowed to pass 

 into the surface-condenser, H. This condenser is 

 full of small tubes, through which a constant 

 current of cold sea-water is kept flowing by means 

 of a circulating pump, J. The steam from G 

 coming in contact with the outer surface of these 

 tubes, is condensed ; and an air-pump, K, draws it 

 off in the form of distilled water along with the 

 air, which, from one cause and another, may have 

 found its way into the engine. The air is simply 

 discharged into the atmosphere, while the water 

 is delivered into the boilers again by the feed- 

 pump, L. The same water is thus continually 



