1840.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



101 



however, given in columns 21 and 22, under the head Comparative 

 fcoiiomical results. 



The next section, which treats of t/ie proportion nf boilers to engines, 

 in our opinion serves ratlier to confuse and perplex tlie reader, and 

 to deprive him of confidence intlie numbers set down in the table, than 

 to render him any assistance in drawing practical conclusions from 

 them. We had intended to make a few observations on particular 

 parts of this section; but having vainly endeavoured to follow the 

 intricate reasoning of the second phragraph, and finding nothing of 

 any importance in the rest, we shall merely direct attention to column 

 20, which will appear on the slightest examination to throw no light 

 whatever on the economical qualities of either boilers or engines. 



In tlie observations on the exptrinitnts and t/uir results, which follow 

 this section, there is nothing worthy of notice before the Gist page, 

 from which we quote the following paragraph, in order to shew how 

 necessary it is to sift with the utmost care all the results tabulated in 

 this work. 



" It is necessar}', also, to guard against conclusions which might be de- 

 duced, from a comparison of the effects of the Cornish engines in the table, 

 with the pressures on the piston and degrees of expansion, set down in 

 columns 5 and C. The pressures given were not ascertained by any instru- 

 ment, (excepting at Huel Towan,) and must he considered only as estimations, 

 not as facts. The pressure upon the piston during the interval which occurs 

 between the first admission of steam into the cylinder, and the instant of 

 shutting it off, may be very variable ; that it was so, in several engines to 

 which Mr. llenwood applied the indicator, is evident from the diagrams he 

 has given, annexed to his paper. (Trans. Inst. C. E. Vol. II.) At the lluel 

 Towan engine, when the steam in the boilers was at a pressure of t/'l lbs. 

 above the atmosphere, it varied from 12-3 lbs. to 7-3 lbs. per square inch on 

 the piston, during its admission into the cylinder ; which latter was its elastic 

 force, at the instant of closing the steam-valve. 1 adduce tliese facts, with 

 the view of showing the impossibihty of determining the precise amount of 

 prssme on the piston, from the degree oi irire-rlrmr'mg the steam ; and as a 

 caution against expectations of deducing any valid theory of the action of 

 the steam, in these Cornish engines, from the particulars of pressure and ex- 

 pansion, contained in the table, which are only approximations to the truth." 



Mr. Parkes does not seem to have compared the numbers contained 

 columns (> and 14, otherwise he never could have considered the 

 above caution necessary ; for the anomalies which would be found to 

 result from the adoption of the numbers there set down are so striking 

 that it would soon be discovered that either the pressure of steam on 

 the piston, or the consumption of water as steam is incorrectly given; 

 and it would certainly not occur to any one to deduce any theory of 

 the action of the steam from such conflicting data. A superficial exa- 

 mination of experiments 7 and 9 will give an idea of the confidence 

 whicli can be placed in the numbers contained in the table. 



The diameters of the cylinders of these two engines are equal, but 

 the latter has four inches greater length of stroke ; the steam is also 

 admitted into the cylinder of the latter during one-fourth of the 

 stroke, while in the former it is cut oft" at one-fifth; but, since the 

 latter only makes 4*29 strokes per minute, while the former makes 

 5"35, the volume of steam consumed in an hour should have been 

 about equal in the two experiments. Now the ]n'essure of the steam 

 before the expansion is given as 7-3 lbs. per sqirare inch in the former 

 case, and 27 lbs. in the hitter case, above the atmosphere, and the 

 volume of steam generated from a given volume of water under these 

 two pressures is respectively proportioned to the numbers 1 173 and 

 Go3 ; the consumption of water as steam must therefore be nearly 

 inversely as these two numbers, and taking the consumption per hour 

 in the former experiment at 2ir)G-21 lbs., as in tlie table, the consump- 

 tion in the latter experiment ought to be about 3s73-2r> lbs., whereas 

 it is given in the table as only 972"G2, or very little more than one- 

 fourth part of what it ought to be. We conclude from this that the 

 numbers set down in column G are of no value whatever, as they do 

 not appear to represent the true pressure on the pistons : nor indeed 

 is it probable that the steam should lose so much as 42 lbs. of its 

 pressure in passing from the boiler to the cylinder, as in the case of 

 the Huel Towan engine, experiment 7. (See columns G and 7.) If 

 wire-drawing is really carried to such an extent in the Cornish en- 

 gines, it is a proof of sad mismanagement ; for, if it is necessary to 

 throttle the steam to such a degree, in order to reduce it to the desired 

 pressure in the cylinder, it is very evident that the load on the safety- 

 valve might be diminished, and the steam thus generated at a lower 

 temperature, the advantages of which are too obvious to need point- 

 ing out here. 



Jlr. Parkes does not seem very confident of the advantage of the 

 Cornish (expansive) system of using steam in manufacturing engines 

 requiring uniformity of motion, and seems to approve of the method 

 recommended by Mr. Wicksteed in such cases, — (see the Journal for 

 January,) namely to employ a Coraish engine to raise water up on a 



wheel, and thus transfer its power to machinery. Now, although the 

 momentum of such machinery is but trifling, an equivalent is easily 

 found in a fly-wheel, and the want of uniformitv in the action of the 

 steam is probably not so great as may be supposed. 



We agree with the author that the pound of water as steam con- 

 sumed by an engine is the most convenient and correct standard of 

 duty which can be adopted, provided we know the true quantity of 

 water which passes through the engine in the form of steam. 



The first division of this work is concluded with a chapter on the 

 Constituent Heat of Steam, \n which the author describes a series of 

 experiments made by himself, the results of which confirm the already 

 generally admitted law, that eqiud weights of water absorb equal 

 quantities of heat in passing from the liquid to the elastic form, under 

 all pressures. 



The remaining portion of this work, which treats of the Locomotive 

 Engine, being very long and perplexing, we have not sufficient leisure 

 to enter into a detailed examination of all the difficulties and doubts, 

 opinions and arguments contained in it ; we must, therefore, content 

 ourselves with a few general remarks. 



The greater part is occupied by an examination of the experiments 

 of M. de Pambour, Mr. Robert Stephenson, Mr. Nicholas Wood, and 

 Dr. Lardner; the object of this examinution being apparently to con- 

 vince the reader of the inaccuracy of some of the results of experi- 

 ment, and of alt the deductions hitherto drawn from them, and to pre- 

 pare him for the reception of a we/y //if ori/ of his own, which he lays 

 down in a separate section near the end of the work. 



It is very certain that the experiments hitherto made on locomotives 

 are too few in number, and too imperfect in their nature to allow of 

 any certain theory of their action being as yet deduced from them ; 

 but on the other hand we have no doubt that a careful investigation 

 of Mr. Parkes' objections would bring many fallacies to light, which 

 might otherwise have the eft'ect of unjustly shaking our confidence in 

 the results previously obtained and published by other authors. As 

 an instance we shall merely cite the comparison he has instituted be- 

 tween two of M. de Pambour's experiments, (pages 9a and follow- 

 ing,) wliich were made with the same engine (AtlasJ at two different 

 speeds, and with corresponding loads. Mr. Parkes, in his detailed 

 calculation of the ettects produced in these two cases, omits, without 

 assigning any reason for so doing, to include the pressure on the 

 back of the piston, which is undoubtedly a part of the resistance, and 

 therefore the power expended in moving this resistance at the ve- 

 locity of the piston, is a part of the gross power of the engine, or of 

 the total effect of the steam. With this omission Mr. Parkes finds the 

 absolute (or gross) power of the steam equal to G7'1I horse power in 

 the first case, where the velocity was 20-34 miles an hour, and 59' 50 

 in the second, where the velocity was 27-09, (See page 9.5.) 



In these two experiments M. de Pambour gives the same effective 

 evaporation, namely, -77 of a cubic foot of water per minute ; and it 

 is to prove the impossibility of this fact that Mr. Parkes made the 

 above comparison; for he observes (page 99) ; "To be consistent, 

 however, with his own (M. de Pambour's) rule above quoted, viz. 

 that ' the weights of water consumed as steam are to each other as 

 the resistances against the piston,' it is obvious that if, in the first 

 case, 302(i lbs. of steam passed through the cylinders in an hour, 

 2 IGG lb. only would have been expended in the second case." Now 

 this assertion is not even justified by his own calculations, for M. de 

 Pambour evidently did not, nor could he mean to say that the weight 

 uf steam which passes through the cylinder in a given time is propor- 

 tional to the resistance, whatever may be the speed of the engine, which 

 would obviously be absurd, but that the density of the steam, and tliere- 

 fore the weight which passes through the cylinder in a given number 

 of strokes, or which is the same thing, in travelling over a given 

 distance, is proportional to the resistance. The consumption of water 

 in a given time would thus be directly as the product of the resistance 

 by the velocity, or the gross power of the engine ; so that, if this 

 power is equal in the two cases, so ought also the evaporation per- 

 minute. 



The effect of the steam in overcoming the resistance of the atmos- 

 phere on the opposite side of the piston is equal to 25"25 horse 

 power in the first case, and to 3G-77 in the second, wliich, added to 

 G7-11 and 59-5U, found by Mr. Parkes, respectively give 92-3 J and9G-27 

 horse power as the gross etti?ct of the steam in the two cases. The 

 near coincidence of these two numbers shews that in this case at least 

 there is no validity in Mr. Parkes' objections. We should therefore 

 recommend a most diligent and patient examination of this section, 

 before the adoption of any opinion therein expressed, or the rejection 

 of any others previously entertained. 



We have already alluded to a }iew tlieonj of the locomotive engine 

 proposed by the author of this work : the section in which he ex-" 

 plains this theory commences (page 124) thus : 



