30G 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[September, 



from the partial resistances siiftered by tlie engine, according to the 

 Trealixi oil Locomolire Eiigiiita ; and as, in the fiist edition of that 

 work, tlie author had confined himself to mentioning the [iressiire 

 against the piston due to the action of the blast-pipe, without making 

 any experimental research on the subject, Mr. Parkes, without noticing 

 the results presented since in the thortj of Oit, sUam etighie, (page 161), 

 takes the dilTorence between the two results, as necessarily exjiressing 

 the pressureilue to the blast-pipe (pa<;es Si?, 83) ; and he demonstrates 

 the inaccuracy of it. Here we perfectly agree with him ; for, besides 

 the errors already pointed out in his research of the pressure of the 

 steam in tlie cylinder, every thing varial)le that can occur in the dif- 

 ferent data of resistance, now passes to the account of the pressure 

 due to the blast-pipe, and must necessarily come to falsify the calcula- 

 tion of it. Thus, for instance, in the experiments, a great deal of 

 water was lost by priming, and there resulted an apparent vaporiza- 

 tion greater than the true one. A part of the difference between the 

 Ciilculated and the observed pressure was tlierefore to be attributed to 

 that cause, though it could not be accurately measured ; but, by the 

 calculation of Mr. Parkes, it all passes to the account of the pressure 

 due to the blast-pipe. Similarly, the resistance of the air, then im- 

 perfectly computed in the total resistance for an average velocity of 

 about \i miles per hour, is found, in all eases of greater velocity, to 

 augment considerably the pressure due to the blast-pipe, and on the 

 contrary to diminish it in all cases of less velocity. A favourable or 

 an unfavourable wind necessarily produce similar effects. Thus, cir- 

 cumstances, combined with the fundamental errors already introduced 

 in the calculation, raise or lower that pressure to all imaginable degrees 

 (pages S7, bS, 90, '.H) ; and it will be readily imagined that such a de- 

 termination cannot be exact. 



4th. Mr. Parkes has observed, in the experiments of the Treatise on 

 Locomotive Eiigims, and particularly in two of them, made with the 

 Leeds engine, and quoted in the Tlitory of the Steam Engine, that the 

 useful effects produced by the same cjuantity of water va[iorized varies 

 according to different circumstances, and he is amazed at it ; for, as 

 he affirms, the useful effects produced by the same quantity of water 

 vaporized, in the same time and under the same pressure in the boiler, 

 ought in all cases to be identical (pages 104, 110). But this again is 

 merely an error of the critic; for if we suppose a locomotive engine 

 drawing a heavy load at a small velocity, since it is only at a small 

 velocity that the engine has to overcome its friction, as well as the 

 atmospheric pressure against the piston, and, above all, the resistance 

 of the air against the train, it follows that out of the quantity of total 

 work executed, there will be but a trifling portion lost in overcoming 

 those resistances; but if, on tlie contrary, we suppose the same engine 

 performing precisely the same cjuantity of total work, but drawing a 

 light load at a great velocity, it is obvious that a much greater part of 

 the work done will be absorbed in moving, at that velocity, the resist- 

 ance which repiesents the friction of the engine, as well as the atmo- 

 spheric pressure against the piston, and in overcoming the resistance 

 of the air, which increases as the square of the velocity; and conse- 

 quently there w ill remain a much smaller portion of it applied to the 

 producing of the useful effect. Hence, in the two cases considered, 

 the useful effects produced by the same quantity of water vaporized, 

 so far from being identical, will, on the contrary, be very different 

 from each other. Mr. Parkes may, besides, satisfy himself on this 

 point, by perusing the Thtorj of the Steam Engine, in which he will 

 find numerous examples of steam engines, in wliich the useful effect 

 of one cubic foot of water varies in very wide limits, according to the 

 velocity of the motion or the load imposed on the engine ; and lie w ill 

 find it explained tlii'oretioally in chapter XII. of the Treatise on Loco- 

 motive Engine--, or in chapter III. art. 11, of the Theory of the Steam 

 Engine. '1 hus Mr. Parkos's reasoning errs again by llie basis itself. 



5tli. But there is another jjrinciple to which Mr. Parkes would sub- 

 ject all llie observations of vaporization of locomotive engines. He 

 remarks that in the two e>.pe]iments above cited, the total resistance 

 opposed to the motion is di!i(?rent in the two cases. Consequently, 

 says lie, the quantities of water vapoiijied iiy the engine in the same 

 time must be ii» proportion to the pressures" in the cylinder, and the 

 exijcviments ought to satisfy this condition (pages 99, 100). Upon 

 tills point he is merciless. 



To establish this new principle, Mr. Parkes recurs io iho Treatise 

 on Locomotivt Engines itself. He quotes a passage in which, suppos- 

 ing same engine travi-Uing the same distance with two different loads, 

 the author says positively that the distance travelled being the same 

 in both cases, the number of turns of the wheel, and consequently the 

 number of strokes of the piston given by the engine, that is to say, Uic 

 number of cylinders full of steam, or firially the total volume of steam 

 exjiended, w.ll also be the same in bothcises; whence results tliat 

 the same volume will successively have been filled with two steams at 

 different pressures, or in other w'ords, at dillercnt densities; and con- 



seqnently the quantities of water which have served to form tliose 

 steams will be in proportion to their respective pressures (page 310 — 

 312 of the first edition). Thus, this passage establishes very distinctly 

 that the quantities of water vaporized, /or the same dintaiice, are in 

 proportion to the pressures of the steam in the cylinders. But what 

 does Mr. Parkes conclude from this? Why, that the quantities of 

 water vaporized in the same distance are in proportion to the pressures 

 in the cylinder. Now it happens to be just the contrary ; for, if we 

 su|)pose, by way of example, the two pressures to be in the ratio of 2 

 to 1, the volumes of water vaporized for the same distance tr.iversed, 

 will also be in the ratio of 2 to 1 ; but if the time employed in per- 

 forming the distance in question be two hours in the first case, and one 

 hour in the second, it is plainly the quantities of water vaporized in 

 two hours and in one hour resnectively, which will be one to the other 

 in the ratio of 2 to 1, so that the vaporizations per hour, or in the same 

 time, will be equal instead of being in the ratio of the pressures. Thus 

 it is clear again that Mr. Parkes's principle rests but on a new error, 

 which consists in making a confusion between the vaporization for the 

 same distance and the vajjorization for the same time. 



Gth. A final observation of Mr. Parkes (pages 89, 90, 9S), is this, 

 that in some experiments, the locomotive engines produced, for the 

 same quantity of water vaporized, a greater useful effect than several 

 stationary high-pressure steam engines, or even than several condens- 

 ing steam engines; and he considers this result as a proof of the in- 

 accuracy of those observations; for, says he, the locomotive engines 

 having to contend with the pressure arising from the blast-pipe, which 

 the high pressure engines have not, and also with the atmospheric 

 pressure, neither of which resistances the condensing engines have to 

 contend with, it is incontestable that they cannot even produce equal 

 eflTects, much less superior ones (page VH). But this reasoning is as 

 unfounded as those we have already noticed; for, since the useful 

 effect of steam engines, for the same vaporization, diminishes as the 

 velocity of the motion increases, which has been already explained 

 above, and which is found developed, either in chapter XII. article II. 

 of the Treatise on Lo'-omotive Engines, second edition, or in chapter 

 111. article II., section 1, of the Theory of the Steam Engine, it is easy 

 to conceive that a locomotive working, for instance, at its maximum 

 useful effect, that is to say, with its maximum load, and consequently 

 at a very small velocity, at which the pressure due to the blast-pipe 

 and the resistance of the air are nearly null, can produce a useful effect 

 greater, nay much greater than a stationary high pressure engine, 

 working on the contrary with a light load and a great velocity. The 

 same inferiority of effect may also occur in a condensing engine, be- 

 cause an engine of that system working, fur instance, at 10 tb. pressure 

 per square inch in the cylinder, and condensing the steam to 4 It), per 

 square inch uniler the piston, where the pressure is always greater than 

 in the condenser, loses, by that fact alone, a quarter of the pow er which, 

 it applies; whereas a locomotive, working at 5 atmospheres in the 

 cylinder, and at a very small velocity, which renders almost null the 

 pressure due to the blast-pipe, suffers, by the opposition of the atmo- 

 spheric pressure, a loss equal to only i of its total power. Hence, de- 

 finitively, in the latter engine, the counter-pressure against the piston 

 destroys a smaller portion of the total power applied, and consequently, 

 wuhout even noticing the difference of friction of the two engines, or 

 enteiing into any other consideration relative to the velocity, it is con- 

 ceivable that the useful effect of the locomotive may be found gieater. 



But if a more complete calculation be desired, it will be easy to 

 furnish it; for the relative volume of the steam at 10 lb. pressure per 

 square inch, being 1072 times that of water, it is plain that if 8 repre- 

 sent the number of cubic feet of water va|)orized ])er minute in the 

 boiler, and if a represent the area of the cylinder expressed in square 

 feet, lo72 S will be the volume of the steam generated per minute 



whence results that will be the velocity, in feet per minute, 



a 



assumed by the piston of the engine working at that pressure. More- 

 over, the effective pressure of the steam or the load which the piston 

 can support, is 10 — 4= 121b. per square inch ; which gives 12 X 144 a 

 for the total resistance, in pounds, supported by tlie piston. Thns, in 

 the condensing engine, the effect produced by the number S of cubic 

 feet of water, expressed in pounds raised one foot per minute, is 

 1072 X 12 X 144 S = 2,&!39,21G S. Calculating in the same manner 

 the case of the locomotive engine, we find that the effect it produces 

 for the same vaporization S, working at the total pressure of 75 lb. per 

 square inch, or at the effective pressure of GO lb. per square inch, and 

 expressed in pounds raised 1 foot per minute, is 3S1 X oU X 144 S=: 

 3,29i,b40S. Therefore, finally, its useful eff<?ct, ;>er cubic foot of 

 water vaporized, will exceed that of the condensing engine, and this 

 again is a circumstance, examples of which will be found in the Theory 

 of the Steam Engine. 



