CHAMBERS'S INFORMATION FOR THE PEOPLE. 



It expresses, however, the total work done by the 

 steam on the piston, and does not shew 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, especially by Mr Richards. 

 The indicator consists essentially of a small 

 cylinder, communicating directly with the cylinder 

 of the engine, and fitted with a piston. It is so 

 arranged that this piston is stationary when the 

 pressure in the cylinder is equal to that of the 

 atmosphere, and rises or falls according as the 

 steam-pressure is higher or lower. A pencil con- 

 nected with the piston is kept in contact with a 

 piece of paper fixed on a vibrating drum, and 

 traces on it a curve, called an indicator diagram 

 or 'card,' from the form of which the engineer 

 can see accurately the pressures at all the different 

 parts of the stroke, and the whole action of the 

 steam in the interior of the engine. 



By taking the mean pressure per square inch 

 on the piston throughout the stroke (deduced from 

 the indicator card), and multiplying it by the area 

 of the piston, and by the number of feet passed 

 through by it in a minute, we should find the 

 number of foot-pounds of work done by the engine 

 per minute ; and this, divided by 33,000, would 

 give the indicated horse-power. 



'Duty' is a measure of power used only for 

 pumping-engines, and differs from horse-power in 

 being entirely independent of time. It is the 

 number of foot-pounds of nett work resulting from 

 the consumption of a given quantity of coal, 

 usually either a bushel of 94 Ibs. or a hundred- 

 weight. At the beginning of this century, the 

 maximum duty that had been attained by any 

 Cornish engine was 20 millions of foot-pounds 

 per cwt. of coal, but six times that duty has since 

 been occasionally obtained. In these engines, it 

 is the actual nett work done which is taken into 

 account ; the duty would be 20 or 25 per cent, 

 greater if the total load on the steam-piston had 

 been considered instead. 



The duty of a Cornish engine is also the 

 measure of its economy, and a somewhat similar 

 standard is used for portable engines. At the 

 Royal Agricultural Society's annual trials in 1872, 

 a portable engine made by Messrs Clayton 

 and Shuttleworth (having a locomotive boiler, 

 and one cylinder 6 inches diameter by 12 

 inches stroke) developed a duty equal to almost 

 79^ million foot-pounds per cwt. of coal. For 

 engines whose power can only be measured by 

 the indicator, the standard of economy is the 

 number of pounds of fuel used per hour per indi- 

 cated horse-power. In factories where ' dross ' is 

 used as fuel, with horizontal engines and Cornish 

 boilers, and where no proper means are taken to 

 insure economy, it is not uncommon to burn from 

 1 5 to 20 Ibs. of fuel per ind. horse-power per hour ; 

 but if the boiler and cylinder are properly covered, 

 and the engine worked expansively, this is reduced 



to from 8 to 12 Ibs. of the same fuel In marine 

 engines and other cases where the best coal only 

 is used, and where high pressures, surface con- 

 densation, and compound cylinders are employed,, 

 the consumption of fuel is often as low as 2 Ibs. 

 per ind. horse-power per hour. 



It is difficult to speak with certainty of the 

 future of the steam-engine. As yet, no signs have 

 arisen of any agent which can be used as a sub- 

 stitute for steam. Gas, air, and air mixed with 

 steam, have all been tried, but hitherto without 

 any such success as to have displaced their rival 

 in the smallest degree, and it seems probable that 

 the steam-engine may hold its own for an in- 

 definite period. It is almost as difficult to form 

 an adequate conception of the advantages already 

 resulting from the use of the steam-engine, as it is 

 to imagine those that are yet to result from its 

 perfection and more extended employment. Navi- 

 gation, travelling, automatic factory labour, print- 

 ing, mining, and a hundred other arts, have been 

 brought to their present state by means of this 

 wonderful agent, and have in their turn reacted 

 upon, and improved it Engineering has now 

 become a science, and improvements can no 

 longer be made empirically. The great mechani- 

 cians who have done so much for the present race 

 of engineers Stephenson, Maudslay, Clement, 

 and many others did their work completely and 

 thoroughly from their own point of view, and mere 

 mechanical improvements on their work come 

 very gradually. Our increased knowledge of the 

 properties of steam and the laws of heat, as well 

 as of the strength of materials, has given a 

 new direction to recent improvements, which are 

 scientific more than mechanical in substance, and 

 scientific rather than empirical in method. We 

 have had bequeathed to us a machine that will 

 do its work smoothly and without breakages, and 

 in which the mechanical means are well adapted 

 to their respective purposes. Our business is now 

 to make every part proportionate, without either 

 lack of material in one place, or superfluous weight 

 in another ; and to apply all our scientific know- 

 ledge to obtaining a maximum efficiency from 

 every part of the engine to make our fuel evapor- 

 ate as much water as possible, to make our steam 

 do as much work as possible, to let our engine 

 absorb as little power in itself as possible ; in fact, 

 to obtain a maximum of economy and simplicity 

 with a minimum of cost In its adaptation to mills 

 and factories, steam is doubtless more costly than 

 water-power ; but being independent of situation 

 or seasons, it is in almost all circumstances prefer- 

 able. Its steadiness, and the ease with which it 

 can be managed and regulated, are also grea 

 recommendations in its favour. As a motive 

 power in the industrial arts, steam takes the pre 

 cedence of all others ; and viewing it as 

 economiser of labour and time, it must 

 assuredly pronounced the greatest of modern 

 mechanical contributors to human progress and 

 comfort 



