THE STEAM-ENGINE. 



board ship. Many ingenious arrangements of the 

 machinery in paddle-steamers have been devised, 

 notably by Maudslay ; but on the whole the oscil- 

 lating engine held its own until the screw-propeller 

 virtually superseded the paddle-wheel. Oscillat- 

 ing engines with inverted cylinders have been 

 used for working screws, but not successfully, 

 partly because of the high speed at which the 

 propeller shaft must run, and partly because along 

 with the screw high pressures of steam came into 

 use, and there are many practical difficulties in 

 the way of using high-pressure steam in oscillating 

 cylinders. 



With the invention of the screw-propeller, every- 

 thing was speedily changed. For a few years 

 screw-engines continued to be the same ponder- 

 ous, slowly-moving machines that had been used 

 to work paddle-wheels, and toothed gearing was 

 used between the crank and the screw shafts to 

 increase the speed of the latter. These, however, 

 were soon abandoned, and direct-acting engines 

 took their place. Trunk-engines were often used, 

 and also engines in which each cylinder had two 

 piston-rods passing right across the shaft (one 

 on each side of it), and attached to the connecting- 

 rod at their further ends. This rod thus occu- 

 pied the side of the vessel opposite to that in 

 which the cylinder was placed ; and from its 

 working back towards the shaft, this form of 

 engine is called the ' return connecting-rod ' type. 



In vessels of war, where it is essential that all 

 the machinery should be kept low in the ship, this 

 form of engine is still used. Although more com- 

 plicated than the trunk-engine, it is more eco- 

 nomical, for much heat is lost from the large 

 surface presented to the air by the trunk when it 

 is out of the cylinder, as it must be at every 

 stroke. In merchant-vessels, however, and all cases 

 where there is no necessity for the machinery 

 being kept low down in the ship, the form known 

 as the ' steam-hammer ' engine is now almost 

 universally adopted. These engines derive their 

 name from their resemblance (in their earlier 

 forms) to Mr Naismith's steam-hammer, the form 

 of which seems to have suggested their arrange- 

 ment. They are direct acting, but the cylinders 

 are inverted, and placed right above the shaft 



The two greatest improvements of modern 

 times namely, the surface-condenser and the 

 compound engine having been brought to per- 

 fection chiefly in connection with marine engines, 

 we may mention them in this place. In sea-going 

 steamers, it is obvious that the only water obtain- 

 able in sufficient quantity for condensing purposes 

 is the salt water surrounding the vessel. The air- 

 pump, therefore, will discharge into the hot well 

 a mixture of sea-water and condensed steam, the 

 former largely preponderating. The feed-pump 

 supplies the boiler with water drawn from the 

 hot well, which must therefore be largely impreg- 

 nated with salt. As the salt is not vaporisable, it 

 will gradually accumulate in the boiler, and form 

 a hard crust on the tubes and furnaces. This 

 crust not only interferes, on account of its bad 

 conducting qualities, with the efficiency of the 

 heating surface ; but its accumulation also forms 

 a source of danger, as the iron underneath it is 

 liable to become red hot, and so give way and 

 cause an explosion. To avoid this latter danger, 

 and as far as possible to prevent the formation of 

 4 scale ' (as the crust is called), it is necessary fre- 



quently to 'blow off' from the boiler a quantity 

 of saturated brine. As this brine has been raised 

 to the full temperature of the steam before it is 

 blown off, it is obvious that the practice, though 

 necessary, wastes an immense quantity of heat. 

 The object of the surface-condenser is to avoid 

 this waste. In it the steam is condensed by con- 

 tact with the surface of a great number of small 

 tubes, through which a current of cold water is 

 kept constantly flowing. By this means, the con- 

 densing water and the condensed steam are kept 

 separate, the former being returned to the sea, 

 while the latter only is sent into the hot well 

 The boiler therefore is continually fed with distilled 

 water. 



In spite of its obvious advantages, the progress 

 of the surface-condenser in public favour has been 

 but slow, partly on account of popular prejudices, 

 partly because of constructive difficulties in the 

 way of carrying out practically what was demon- 

 strably true in theory. The same may be said 

 about the compound engine. These difficulties, 

 however, are now almost entirely surmounted, and 

 compound surface-condensing engines, worked at 

 what would, only a few years ago, have been con- 

 sidered to be dangerously high pressures, bid fair 

 shortly to be universally adopted in steam-vessels 

 where it is desired to attain any considerable 

 degree of economy. 



In compound engines, the cylinders are of 

 unequal size the larger, called the low-pressure 

 cylinder, being from three to four times the 

 capacity of the smaller, or high-pressure cylinder. 

 The steam from the boiler is admitted into the 

 latter in the usual way, and cut off at or 

 stroke ; and after doing its work there it is con- 

 ducted to the large cylinder (where its reduced 

 pressure, by acting on an increased area, does as 

 much work as in the other cylinder), and thence 

 to the condenser. If the large cylinder has four 

 times the capacity of the small one, and the steam 

 is cut off at stroke in the latter, it is obvious 

 that it will be expanded eight times. In cases 

 where a fly-wheel cannot be used, this has many 

 advantages over an engine in which the steam is 

 expanded eight times in a single cylinder. An 

 engraving of a compound marine engine is given 

 in the article MARITIME CONVEYANCE. 



The Work done by Steam-engines. This is esti- 

 mated in two ways as horse-power, and as duty, 

 and the first expression includes two things 

 nominal, and indicated, horse-power. Thirty- 

 three thousand foot-pounds of work done per 

 minute is called one horse-power, this being con- 

 sidered by Watt as the maximum force which a 

 strong horse could exert. The nominal horse- 

 power of an engine has long ceased to be any 

 expression of the actual power it exerts ; it is 

 only used as a kind of commercial standard (a 

 very deficient one) for the sale and purchase of 

 engines. It is generally made to depend entirely 

 on the diameter of the cylinder, but the piston 

 area allowed per horse-power is alike in no two 

 of the great manufacturing centres. For hori- 

 zontal non-condensing engines, about 9 or 10 

 square inches, and for marine engines, about 24 

 or 25 square inches of piston area are allowed 

 per nominal horse-power by most engineers in 

 Scotland and the north of England. 



The indicated horse-power is the most useful 

 measure we have of the work done by an engine. 



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