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ST i :. \.M-KM; INK 



the opening C is put into communication with the 

 boiler, the opening D with the atmosphere, and 

 the steam immediately rushes out of tin- <> limlri, 

 and dissipate* its contained energy through the 

 air. Although the steam, when allowed to go 

 into the atmosphere, is immediately reduced to 

 the pressure corresponding to the temperature 

 of the air (which in ordinary cases would be 

 only a fraction of a pound per square inch), 

 still the full pressure of the atmosphere iteelf 

 will always be acting on the back of the 

 piston during its stroke, and therefore, to find 

 the force with which the piston is being pushed 

 along, we must subtract that pressure from the 

 steam-pressure. On the one side of the piston 

 will be the atmosphere with its uniform pressure 

 of nearly 15 Ib. per square inch, and on the other 

 side the steam-pressure of 60 Ib. The effective 

 pressure thus will be 60 - 15, or 45 Ib. per square 

 inch only. 



Let us now consider the somewhat more econom- 

 ical case of an engine in which the steam is first 

 used as described above, hut afterwards, instead 

 of being allowed to pass into the atmosphere, is 

 conducted through a pipe into a closed vessel and 

 there condensed. Condensation consists in the 

 subtraction from steam of a portion of its sensible 

 heat. This reduction of temperature has a double 

 effect on the steam : ( 1 ) the cooling and lique- 

 faction of a part of it; and (2) the reduction of 

 the rest to the pressure corresponding to the 

 reduced temperature. It is not possible to do 

 one of these things without the other. What 

 is commonly called ' vacuum ' simply means 

 pressure less than the atmospheric pressure ; 

 and, in the case of steam-engines, a vacuum 

 generally implies a pressure of between 2 and 

 3 II). per square inch i.e. from a seventh to a 

 fifth of the ordinary pressure of the air. The 

 most common way of condensing steam is by 

 bringing it into contact either with a jet of cold 

 water or with surfaces kept continually cool 

 by a current of water. In either case, directly 

 the steam is brought into contact with the 

 water or cooling surface, it transfers to it the 

 larger portion of its sensible heat. During this 

 process the greater part of the steam is liquefied, 

 and the remainder retains only such a pressure as 

 corresponds to its greatly reduced temperature. 



The advantages possessed by a condensing over 

 a non-condensing engine will now be obvious. 

 When the piston is being forced from C to D by 

 steam entering through C, the force on the back of 

 the piston resisting its motion in that direction, 

 instead of being equal to the pressure of the atmo- 

 sphere, is only the pressure of the steam in the 

 condenser, or about 2 Ib. per square inch. The net 

 effective force is therefore 60 - 2 or 58 Ib. instead 

 of 60 - 15 or 45 Ib. 



We have supposed that our cylinder when full 

 of steam contain"! just one pound- weight at 60 Ih. 

 pressure. Let us now find out how much useful 

 work this pound of steam has done for us, and we 

 will then show how the same weight may be mode 

 to do a great deal more, by utilising more of its 

 great store of heat. Let us suppose that the area 

 of the cylinder is 2 square feet, while its length 

 (the stroke of the piston) is :t l , feet. It will thus 

 have a capacity of 7 cubic feet, as liefore assumed. 

 In the first cose described we should have a pres- 

 mire of 45 Ib. per square inch exerted on an area of 

 28S square inches through a distance of 3A feet. 

 This is equal to 45,360 foot-pounds of work. In 

 the second case we have a pressure of 58 Ib. per 

 square inch on the same area and through the same 

 distance. This is equal to 58,464 foot-pounds of 

 work, or about ,V ' the total heat supplied by 

 the fuel. (For simplicity's sake we have here 



assumed that the water in the boiler has to be 

 raised from 32 to 292, and evaporated at that 

 temperature. If the water were supplied at 212, 

 then the work done would !>< about i l j instead of 

 j'i of the total heat.) We may now proceed to 

 examine the way in which the same weight of 

 steam, generated by the consumption of an iden- 

 tical weight of fuel, may ! mode to perform many 

 times more work by ' working expansively.' 



One of the properties possessed by steam, in com- 

 mon with all other gases, is a tendency to expand 

 indefinitely ; its pressure varies nearly inversely as 

 its volume. For simplicity's sake we shall here 

 assume that steam is a perfect gas, and follows 

 Boyle's law, the pressure varying exactly inversely, 

 as the volume. If then we have a cylinder of the 

 same area as before, but of twice the length, but 

 only intend to admit 1 Ib. of steam into it 

 at a time, it will be necessary, when the piston 

 has travelled :U feet of its stroke, to shut the 

 entrance valve, so as to prevent more steam 

 entering; this is called 'cutting off' the steam. 

 The piston, however, still continues its motion in 

 the same direction as before, propelled by the inter 

 nol separative energy among the particles of steam. 

 But as it is pressed forward the space occupied by 

 the steam is always increasing, and its pressure 

 always decreasing in proportion, until at length, 

 when the piston has reached the end of its stroke, 

 the steam occupies exactly double ite original 

 volume viz. 14 cubic feet, and is reduced in pres- 

 sure to half its original pressure viz. to 30 Ib. ]>er 

 square inch. We have tlius during the first half of 

 the stroke a constant pressure on the piston of 60 

 Ib. per square inch, and during the second half a 

 pressure gradually decreasing from 60 to 30 Ib. 

 The mean pressure during this second half of the 

 stroke will be found on calculation to l>e almost 

 exactly 40 Ib. Let us now, in the same way a 

 liefore, see what work we have been able to get out 

 of our pound of steam by expanding it in this way. 

 In the first half of the stroke we have 58,464 foot- 

 pounds of work exactly as before, and then we 

 nave in addition a mean pressure of 40 - 2, or 38 

 Ib. per square inch, exerted over 288 square inches 

 for a distance of 3} feet. This equals 38,304 foot- 

 pounds, muking a total of 96,768 foot-pounds of 

 work obtained from the steam which only gave us 

 58,464 before. The economy of working expan- 

 sively, however, goes much further than this. If 

 the cylinder had lieen four times its original 

 length, and the steam had been cut oil at the 

 same point as l>efore (which would then lie quarter 

 instead of half stroke), we should have obtained 

 from the I Ib. of steam about 144,000 foot-pounds 

 of work. If we hod gone still further and ex- 

 panded the pound of steam into eight times its 

 original volume, we should have obtained about 

 180,000 foot-pounds of work, which is more than 

 three times as much as at first ( In actual work- 

 ing, owing to various causes such as imperfect 

 action of the valves, radiation from the cylinder, 

 bad vacuum, &c. the work obtained from the 

 steam is not more than -lif) to '75 of that given in 

 this paragraph.) All modern engines are worked 

 more or less on this principle of expansion, and the 

 general tendency seems to be every year to adopt 

 higher initial pressures and (within certain limits) 

 larger ratios of expansion. 



Fig. 3 represents Watt's 'double-acting' con- 

 densing engine. By 'double-acting engine" we 

 mean an engine such as was sketched in fig. 2, in 

 which the steam acts on both sides of the piston 

 instead of only on one, as in Newcomen's engine. 

 Watt's engine, though not of the form now gener- 

 ally used, contains all the parts now considered 

 essential. The steam from the boiler passes direct 

 to the valve-chest, v, which is simply a long box 



