140 LECTURES TO SCIENCE TEACHEES. 



densing and expansive. I will first of all take the simplest 

 and worst form of steam-engine, that of non-condensing 

 and non-expansive, and I will just ask you to consider 

 how important it is under these circumstances that the 

 steam which is used should be of very high pressure. So 

 obvious is this that it would seem absolutely unnecessary 

 to labour the point, were it not that one knows m practice 

 steam-engine users, and too frequently steam-engine makers, 

 will employ large cylinders to perform a given amount of 

 work in non-condensing, non-expansive engines, having a 

 notion that they are really doing good, and are at the same 

 time acting liberally, whereas they are doing harm and are 

 making the engine wasteful. The effects in a non-condens- 

 ing and non-expansive engine of using different pressures of 

 steam are shown in Diagram 3. I will assume, to begin 

 with, that we have a pressure of steam in the boiler 120 

 Ibs. above zero, and for simplicity and to get rid of fractions I 

 will take the atmosphere as equal to 15 Ibs. At 120 Ibs. 

 above zero if we use a steam-engine without expansion and 

 deduct 15 Ibs. for the resistance of the atmosphere we get 

 from a given quantity of steam a work of 105. If we use 

 it at only 60 Ibs. above atmosphere, deducting 15 Ibs., we 

 obtain a work of 45, while if we use it at 30, and deduct 15, 

 we obtain a work of 15. If, however, we use it at 18 above 

 atmosphere and deduct 15 we obtain a work of only 3 ; 

 that is to say, the percentage of coal required to raise 

 the steam to atmospheric pressure is clearly greater the 

 less is the pressure of the steam. If we try to work a 

 non-condensing engine with steam at atmospheric pressure, 

 evidently, we should get no work out of it at all, although 

 we should expend the coal in boiling off the water ; if the 

 engine were worked at 1 Ib. above the atmosphere, yfths 

 of the coal would go away in boiling the water and Y^-th 

 only would be utilised. But if worked at 120 Ibs. above zero 

 or 105 Ibs. above atmosphere, fth only of coal is lost, and 

 j-ths are utilised. Therefore we see how essential it is that 

 to obtain in a non-condensing, non-expansive engine any- 

 thing approaching to economy, high pressure steam should be 

 employed. 



I will now ask your attention to the advantage in non- 

 condensing engines of the use of expansion. The results 

 are expressed on this diagram, which exhibits the well-known 





