8^ 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[September, 



])0\vcr having lieen foiinrl equal to the resistance in tlie first case, would he 

 necessarily sujjerior to the resistance in the second. 



7tlily anil lastly. On looking over our experiments on locomotives, the 

 same engine will he seen sometimes drawing a \ery light load with a high 

 pressure in the hoiler, and sometimes, on the contrary, a very heavy load with 

 a low pressure. It is then impossihle to admit, as the ordinary theory woidd 

 liave us, that there is any fixed ratio whatever hetween the two pressures. 

 This ettect, moreover, is most easy to explain ; for it depends simply on this, 

 that in liotli cases the pressure in the hoiler was superior to the resistance 

 against the piston, and no more was needful in order that the steam, 

 generated at that pressure, or at any other fulfilling merely that comlition, 

 might, on passing into the cylinder, assume the pressm'e of the resistance. 



Here the author evidently assumes that a decrease of the load of 

 an engine would not be foUow-ed by a corresponding diminution of 

 pressure in the boiler. But it is clear that this eft'ect can only be 

 prevented by partially closing the regulator. 



All that precedes is, as we liave already stated, contradicted in the 

 following paragraph, which is the last of the section. 



It is, however, essential to observe, that we wish to establish by these 

 reasonings, that, since the pressm'C in the cylinder is fixed a priori, it cannot 

 depend on the pressure of the hoiler; hut we believe, on the contraiy, as 

 will be seen. Sect. VII., that the pressure in the cylinder being once regulated 

 by tlie resistance on the piston, that of the boiler afterwards de))Pnds on it, 

 in jn-oportion to the size of the passages, the volume of steam produced, and 

 the weiglit of the safety-valves. It would only be for want of making this 

 needful distinction, that we could be thought to admit an entire independence 

 between the two pressm-es. 



Sect. VI. — Comparison of the tmo theories in their application to 

 particular examphs. 



The facts here stated require a corroboration before we can admit 

 tliem as evidence of the correctness or incorrectness of any theory. 

 The examples cited are two experiments which are given, pages 233 

 and 234 of Pamhour's Treatise on Locumotivts. 



1. The locomotive engine Leeds, which has two cylinders of 11 inches 

 diameter; stroke of the piston, 10 inches; wheels, 5 feet; weight, 7'07 



tons; drew a load of 81'34 tons, ascending a plane inclined at the 



velocity of 20-34 miles per hour ; the effective pressure in the boiler being 

 54 lbs. per square inch, or the total pressure 68'71 lbs. per square inch. 



2. The same day the same engine drew a load of 38'J2 tons, descending 

 1 



a plane inclined 



1094 



at the velocity of 29-09 miles per hour ; the pres- 



sm-e in the boiler being precisely the same as in the preceding experiment, 

 anil the regulator opened to the same degree. 



We do not believe it possible that, under such circumstances, the 

 pressure in the boiler could be so high in the second Ciise as in the first. 

 But if the pressure were really GS-71 lbs. on the square inch in tlie 

 Hrstcase, we are led to the conclusion that it could not have been more 

 than 41J-44 in the second, to satisfy all the conditions supposed. On 

 this hypothesis we should find, using the coetticient 0-625, in both 

 cases: 



1st Case. Useful effect, by the ordinary calculation, 

 (68-71 X n-625 — 14-71) x 190-08 . 

 Useful ertect produced, from M. de Pam- 

 hour's calculation, page 35 ... 



Difference 

 2nd Case. Useful eft'ect by the ordinary calculation, 

 (46.44 X 0-625 — 14-71.) x 190.08 . 

 Useful effect produced, from M. de Pam- 

 bour's calculation 



Difference . 

 Mean difference 



5367 lbs. 



5404 



37 



2721 



2708 



13 

 12 



Thus, supposing the amount of resistances, as calculated by M. de 

 Pambour to be correct, as well as the pressure in the boiler in the 

 first case, but reducing the pressure in the second ease in conformity 

 witli the law of the flowing of elastic fluids, and making use of the 

 coefficient -625, we commit an error by the ordinary mode of calcu- 

 lation of 37 lbs. in the first case, and 13 lbs, in the second, one plus and 

 the other minus, so that the mean error is 12 lbs. By M. de Pam- 

 bour's method, the error is 404 lbs. in the first case, and 131 lbs. in the 

 second, one plus and the other minus, making the mean error 13GA 

 lbs. But we have no doubt one of the resistances has been estimated 

 too high in the second case, namely, the resistance caused by the 

 blast-pipe. In the first case, tliis was assumed to be equal to 3-4 lbs. 

 on each square inch of the pistons, which we believe to be very near 

 the truth; but in the second case, where the cylinder is filled with 

 steam of less elastic force, the resistance it opposes to its expulsion 

 is taken at 5'1 lbs. per square inch, we calculate it to be about 2'lj lbs. 



per square inch, which makes the sum of resistances in the second 

 case (supposing all tlie rest correct) less by 475 lbs. than in the 

 author's calculation. 'We should thus have in this case, 



KlTort e.xerled by the engine by M. de Pambour's calculation . 7215 lbs. 

 Effect produced, including friction, &c 6871 



Difference .... 344 

 This dilTerence, being on the same side as that in the first case, 

 makes the mean error 374 lbs. 



However, ignorant as we are as to the actual pressure in the boiler, 

 and the exact quantity of steam which passed through the engine in 

 the two experiments, we can oft'er nothing but surmises and hvpotheses 

 on the subject, hoping that all doubt and uncertaintv may shortly be 

 cleared away by more circumstantial and conclusive experiments. 



We would not be understood to approve altogether of the ordinary 

 mode of calculating the power of a steam-engine : we believe the fric- 

 tion of the engine to be generally much exaggerated, and we do not 

 look upon the system ;is perfectly accurate ; yet we cannot admit the 

 proofs brought forward by M. de Pambour as conclusive of the inaccu- 

 racy of this, or the accuracy of his own theory. 



Sect. 'VII. — Of the area of the steam passages. 



We have already mentioned that the author asserts in this section 

 " that the degree of o))ening of the regulator can have no influence on 

 the pressure in the cylinder, but that its reaction, on the contrary, is 

 upon the pressure in the boiler." This he endeavours to demonstrate 

 in the 4Sth and following pages , but we think we can shew from the 

 following quotation that he has miscomprehended the meaning of the 

 expression, that the degree of opening of tlie regulator injlmnces the pres- 

 sure in the cylinder, and that his arguments fully prove that this pres- 

 sure is really influenced by that circumstance. 



It should be borne in mind that, when it is stated that a contraction 

 of the steam passages is accompanied by a diminution of pressure in 

 the cylinder, the velocity of the piston is supposed to be constant ; 

 the author admits that that contraction may change the quantity of 

 steam which passes through it, therefore, a smaller quantity having to 

 fill the same space, its density, and with it its pressure must neces- 

 sarily be diminished. But M. de Pambour objects to this, that the 

 pressure in the cylinder is always strictly determined, a priori, by the 

 resistance on the piston. This is true ; but if the resistance on the 

 piston is not known, neither is the elastic force of the steam, and it is 

 evident that with a given pressure in the boiler, area of steam passages 

 and load, the piston of an engine can only travel at a certain velocity ; 

 so that, if the pressure in the boiler remain the same, bu{ the area of 

 the passages be contracted, the same velocity can only be kept up by 

 diminishing the load or resistance on the piston, in which case the 

 ])ressui-e in the cylinder must also be diminished, according to M. de 

 Pambour's own theory. 



hi concluding our remarks on the first chapter, we will observe that, 

 in our opinion the theory ]iroposed does not differ in any great degree 

 from the ordinary theorv, but in consequence of the opinion enter- 

 tained by the author, that there can exist any difference whatever 

 between the pressure in the boiler and that in the cylinder, the results 

 furnished by the two theories appear to be totally at variance. M. de 

 Pambour's "chief objection to the ordinary theory is to the use of a 

 constant co-elficient ; but if the co-efficient were applied to the eff'ec- 

 tive pressure, instead of the total pressure in the boiler, it would be 

 little more a method of co-efficitnts than that proposed by the author ; 

 for it would become 



r = k{R—p), 

 r being the pressure on each square inch of the piston due to the load 

 or useful effect, R the total pressme in the cylinder (which we may 

 assume to be known, since we suppose that pressure to differ but in- 

 considerably from that in the boiler, and, therefore, consider it a suffi- 

 ciently near approximation to take off a constant proportion of the 

 latter), p is the pressure on the opposite side of the piston, and k the 

 constant co-efficient. By M. de Pambour's theory we find 



in which/ is the friction of the engine without any load. M. de Pain- 



-1 7 



hour estimates the fraction — — - **— -> and/= lib.; and if we as- 

 1 + S 8 



sume R — ;; = 16, for a condensing engine, we shall have, for that 

 particular case,/ = ttj (fl — ;;), and 



r~J i.(R- 



l {(«-i^)-li(«-i')} 



