STEAM-ENGINE 



705 



cylinder. From the back of the firebox numerous 

 small tubes traverse the boiler (through the water) 

 to the smoke-box, /, and conduct the products of 

 combustion to the chimney, g. The steam-pipe, k, 

 is led away from near the top of the dome, h, and 

 fitted with a regulator valve, /. At m are a pair of 



spring safety-valves. Both cylinders discharge their 

 steam through the vertical blast-pipe, p, and by 

 this means a sufficient draught is caused, notwith- 

 standing the small height of the chimney. The 

 cylinders, r, are placed in the bottom of the smoke- 

 box, and partly enclosed in it. 



Fig. 7. 



In all marine engines, except the very smallest, 

 two cylinders are used, working cranks at right 

 angles to each other, so as to equalise the motion 

 as far as possible, it being almost impossible to use 

 a flywheel of sufficient weight for that purpose on 

 board ship. The form originally known as the 

 'steam-hammer' engine (from the resemblance of 

 early models to Nasmyth's steam-hammer), or 

 some modification of it, is now almost universally 

 adopted. They are direct acting, but the cylinders 

 are inverted, and placed right above the propeller 

 shaft. Two of tin- greatest improvements in the 

 modern steam-engine the surface-condenser and 

 the compound engine have been brought to per- 

 fection chiefly in connection with marine engines 

 here. In the surface-condenser the steam is con- 

 densed by contact with the exterior surface of a 

 great number of small tubes, through the interior 

 of which a current of cold sea-water is kept con- 

 stantly flowing. By this means the condensing 

 water and the condensed steam are kept separate, 

 the former being returned to the sea, and the latter 

 only sent into the hot well. The boiler, therefore, 

 is continually fed with distilled water, and the 

 wasteful process of ' blowing off," to get rid of the 

 unvaporisable matter which would otherwise be 

 deposited in the boiler, is rendered unnecessary. 



In ' compound ' engines the two cylinders are of 

 unequal size the larger, called the low-pressure 

 cylinder, having 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 generally at 

 from | to of the stroke ; and after doing its work 

 there, it is conducted 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. This system of 

 engine has several notable advantages among 

 which are that the driving pressures are more 

 uniform than in ordinary engines ; that leakage 

 past the piston becomes of less importance ; that 

 for any given large measure of expansion the 

 mechanism of the engine is much more simple 

 than for the same degree of expansion carried out 

 independently in two cylinders ; and that the losses 

 due to condensation of steam in the cylinders 

 461 



(which are now known to be among the most 

 serious of all causes of waste) are much reduced. 



In modern marine engines, and to some extent 

 also in mill engines, the compound principle is 

 now carried further, and ' triple expansion ' engines 

 (which are simply compound -engines with three 

 cylinders used consecutively instead of two) are 

 very widely employed, with very economical results. 

 In these engines steam is not uncommonly used at 

 a pressure as great as J50 Ib. per square inch, or 

 six times as much as was usual about 1860. Quad- 

 ruple engines are also used. 



The Work done by Steam-engines is estimated 

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

 the first expression includes two things nominal 

 and indicated horse-power. Thirty-three thou- 

 sand foot-pounds of work done per minute is 

 called one horse-power, this being considered by 

 Watt as the maximum rate at which a strong 

 horse can work. 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, and is generally made 

 to depend entirely on the diameter of the cylinder. 

 The indicated horse-power is the most useful 

 measure we have of the work done by an engine. 

 It expresses, however, not the work itself, but the 

 rate at which that work is being done in the 

 cylinder. It has to be remembered also that it 

 does not show at all what proportion of that work 

 has to be expended in overcoming the friction of 

 the engine itself. It is ascertained by the use of 

 a little machine called an ' indicator, ' devised by 

 Watt, and since his time greatly improved, especi- 

 ally by Richards and by the Crosby Company. 

 By taking the mean pressure per square inch on 

 the piston throughout the stroke (measured from 

 the indicator diagram), and multiplying it by the 

 area of the piston and by the number of feet 

 passed through by it in a minute, we can find the 

 number of foot-pounds of work done by the engine 

 per minute ; and this, divided by 33,000, gives the 

 indicated horse-power. 



' Duty ' is an expression used only for pumping- 

 engines, and differs from horse-power in being 

 entirely independent of time i.e. it is a measure 



