134 



THE CIVIL KNGINEER AND ARCHITECT'S JOURNAL. 



[April, 



ship are proiKJitiiiiuitply reduced. Besides an obvious decrease of 

 cost resultiiifr t'vom these improvements, it is manifest tliat the 

 dispensing- with several vvorkinff j)arts, as the parallel motion, 

 beam and its f;udf;eons, connectino--rod, itc, must, to some extent, 

 (by reducing the friction, vis inertia, and momentum), economise 

 power; and we think it requires no great stretch of credulity to 

 believe that smtie economy of fuel must arise by these reductions 

 from the arrangement in the Pimlico engine, and which you report 

 to he working with 2,^ lb. per horse-power per hour — certainly an 

 e.xcellent performance, but not in any way superior to our best 

 engines. 



Having said thus much in justification of our claim to notice, 

 and in confirmation of some of the facts given in the public jour- 

 nals, we think it will not be out of place to advert, as an interest- 

 ing matter of history, to some of your remarks when treating of 

 double-cylinder expansion, especially as regards the first introduc- 

 tion of expansive steam, both in the single and double cylinder, or 

 in what you have termed the "Watt and AV'oolf engines," as well 

 as to some other observations you have made on the subject. 



It ought to be more generally known than it appears to be, that 

 the credit of having first ]iropounded "double-cylinder expansion" 

 is due to Jonathan Hornhlower, and not (as you have assumed, 

 and is very frequently supposed) to Arthur Woolf. Hornhlower 

 patented the system, with ample and efficient details, in 1781; that 

 is to say, twenty-three years before 1804, the year in wliich you 

 have stated Woolf published the discovery. The following ab- 

 stract from Horidjlower's specification will show that he fully de- 

 scribes this species of engine. 



"/'^■.s/, T use two vessels in which the steam is to act, and which in 

 other C)if/ines are called cylinders. Secondly, I employ the steam after 

 it has acted in the first vessel, to operate a second time in the other by 

 ))ernntting it to e.vpand itself, which I do by connecting the vessels to- 

 gether, and forming proper channels and apertures whereby the steam 

 shall occasionally go in and out of the said vessels, S;c." The descrip- 

 tion and illustrations of Hornblower gave a complete arrangement 

 of valves and other details, and rendered the system perfectly 

 practical, so as to leave nothing wanting to the full development 

 of the double-cylinder expansive engine. Most of wliat has since 

 been done is due rather to the progressive advances towards a 

 more perfect system of manipulation, and to that simplification 

 and just proportioning of the parts which experience only could 

 have warranted. What Woolf did was to bring a mind of a highly 

 practical turn to hear on Hornblower's system, and in this he was 

 so successful as to be fully entitled to rank as one of the first on 

 the list of eminent constructors; for, although commencing as he 

 did under a delusion and a fallacy, as regards tlie rate at which 

 steam decreases in pressure while expanding, there is no doubt 

 that it is entirely owing to his ready appreciation of the value of 

 liigh steam when used expansively, and to the practical skill by 

 which he made it available in the mining operations of Cornwall, 

 in despite of practical difficulties and (more formidable still) of a 

 powerful and prejudiced opposition, that Cornish mining has con- 

 tinued to be of its present extent and importance — since, but for 

 the large reduction in quantity of fuel consumed by pumping- 

 engines from what it was in the days of AV'att, many now profit- 

 able mines must have been abandoned or remained unworked, as 

 the cost of fuel would have exceeded the value of the ores, and 

 precluded those further researches which have from time to time 

 led to the discovery of the most valuable mining treasures. We 

 may add also the more important fact, that it was in a great 

 measure owing to the economical results as regards fuel, resulting 

 from 'Woolf's success in Cornwall, that the expansive system has 

 obtained so generally the sanction of our best practitioners, as is 

 evinced by its almost universal introduction. 



The pumping-engines of Cornwall are, with scarcely an excep- 

 tion, constructed on the principle of expanding steam in one 

 cylinder; and you are <juite right in stating, that the double-cylinder 

 system is inferior fm- pumping purposes. 



There is no question that single-cylinder expansion, if the load 

 can duly be proportioned to the effort of the steam from its first 

 impact on the ])iston to its minimum of effective attenuation, will 

 produce a greater absolute impulse, or, as it is termed, a better 

 duty for the v(dume of steam consumed, than if the medium were 

 a double cylinder. It is thus that the ])ower of the single cylinder 

 is given out in tlie pumping-engines of Cornwall; and whereby the 

 consumption of coal has been brought as low as I'T.lll). per horse- 

 power per hour in the best example, for by the introduction of 

 the plunger-])ump, the power of the engine is, when the piston is 

 subjected to the highest steam pressure (that is to say, before the 

 sujiply from the boiler is cut oft), exerted to overcome the vis 



inertife of the pit work, besides its unbalanced weight, the column 

 of water being then at rest. Once in motion, the duty is that of 

 overcoming little more than mere gravity; and ultimately the 

 extreme expansion of the steam, as the piston a])proaches the bot- 

 tom of the cylinder, serves to check the momentum of the pit- 

 work. The column of water is raised on the return stroke, not 

 by the direct eff(n-t of the steam, but by the gravity of the unba- 

 lanced weight of the ])itwork. The piston of the engine ascending 

 in equilibrio, as regards steam pressure, it will readily be perceived, 

 that by these arrangements, the efforts of high steam at, and a 

 little beyond, the commencement of the descending stroke, its sub- 

 sequent expansion as the rw incrtice is being overcome, and its 

 gradual attenuation as it approaches the termination of its course 

 (where the efforts of momentum and pressure should both be 

 exhausted), is better and more simply, as well as more philosophi- 

 cally employed than it could be by the double cylinder ; wherein 

 the main distinctive feature is an approximation to uniformity of 

 effort, and which is, on that account, so far inapplicable to the 

 moving a load presenting the changes of resistance just stated. 



It will he corollary to the preceding conclusions, that the import- 

 ance of preserving a due relation between the power and the load, 

 rendei-s it as desirable that the power of a rotatory machine 

 should preserve its uniformity, as that the power of a pumping- 

 engine, under Cornish arrangements, should be unequal. Hence 

 it is solely owing to this approach to uniformity of effort, that 

 double-cylinder expansion possesses any advantages over the single 

 cylinder whenever the power is employed to produce rotation. 



We believe you will find you are wrong in stating, that single- 

 cylinder expansion "is very commonly adopted in cotton s\nn- 

 ning;" for, on the contrary, if we are correctly informed, the 

 employment of double-cylinder expansion is becoming very exten- 

 sive in the cotton factories; and manufacturers are thereby enabled 

 to spin cotton thread as fine as can be produced by water-power — a 

 result wholly unattainable by single-cylinder expansion. 



It is common to call the fly-wheel a reservoir of power, and it 

 is quite true that it is so; but this propertj', imperfectly under- 

 stood, leads to a popular mistake. The notion that revolving 

 bodies must rotate uniformly, is so closely allied to our impressions 

 regarding circular movement, that it is difficult to divest the mind 

 of the idea that it is otherwise; and hence it is seldom duly con- 

 sidered, that to be a reservoir of power, the fly-wbeel must have 

 an intermittent velocity. 



The fact, however, is, that so far from being, in any instance 

 whatever (as it is frequently supposed) a perfect equaliser of 

 unequal efforts, it is entirely owing to the necessary changes in its 

 velocity that it becomes the reservoir of those excesses of power 

 which arise from unequal impulses; since, as is obvious, such 

 excesses can only be absorbed into the fly-wheel by the fly-wheel 

 acquiring an increased velocity; and that they can only be given 

 out again when required to overcome the load or resistance by 

 losing the momentum due to the increased velocity, and conse- 

 quently losing the excess of velocity the fly-wheel liad acquired. 

 Or it may be more clearly stated thus: the velocity, and conse- 

 quent momentum of the fly-wheel, are conjointly increased or 

 diminished, in an assigned proportion, as either the load or the 

 effect to increase it are in excess. 



We see, therefore, that however the dynamic efforts of expand- 

 ing steam may economise fuel, its great inequality of effort, when 

 given through the medium of a single piston, would forbid us to 

 avail of that property to such an extent as to be of an appreciable 

 practical value in cases where great uniformity of motion, as in 

 cotton spinning, grinding corn, and several other delicate opera- 

 tions, are the prime consideration. 



Even a perfect uniformity of effort on the piston, must, in all 

 cases when applied to a reciprocating engine, entail some inequality 

 of motion in the mill work. Such inequality, however, is reduced 

 to a very small amount, by the employment of a pair of either 

 single-cylinder non-expansive steam-cylinders, or of double cylin- 

 ders acting expansively. We need not add, however, that the 

 double-cylinder system must (as yon have properly shown in your 

 notice of tlic Pinilico engine) prove by far the most economical as 

 regards fuel. We are, &c. 



Greenwich Iron Works, W. Joyce & Co. 



March \9th, 1850. 



*,;.* Our professional brethren will not fail to perceive, that the 

 letter by Messrs. Joyce and Co. is of a compound nature. In 

 part, it is a reply to the remarks we made in our last number; 

 in p.-irt, it is historical, and elucidatory of the action of the fly- 

 wheel, and of the systems of pumping, as adopted in Cornwall; 



