.340 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Skptembkr, 



uitli tliat of some c\])oriiiionts maile on engines of tlie same kind, tlie ratio 

 Ijptwoon the two lesnlts lia<l furnished a fraetional coetiieient, wliich was re- 

 garded as the constant ratio between tlic tlieorctieal and jiractical etfects of 

 all engines of the same system ; therefore, in multiplying the nnndier ex- 

 pressing the theoretic ettect by this fractional coefficient, a dclinitive product 

 was obtained, which was the ^rae^jca/ effect that could be expected from the 

 engine. 



Now, the Ikoretic effect cannot be obtained without first deditcting 

 froni the pressure in the boiler that on the opposite side of the piston; 

 and tlie co-efficient was applied, not to the effective pressure so found, 

 hut to the total pressure in the boiler, the pressure on the opposite 

 side of the piston being regarded as part of the direct resistance over- 

 come, and therefore added to the useful effect. 



In the next paragraph to that just quoted, the same rule is ex- 

 ])ressed algebraically, a constant coefficient being used to pass from 

 the thfontic to the practical effect ; thus, the former is expressed by 

 iiTT, and the latter by kan, a representing the area of the jiistun, 

 7r the pressure in the boiler, and k the constant coefficient. M. de 

 Pambour here gives a table of the coefficients indicated by Tredgold, 

 and in the next follow ing paragraph corrects the above rule, by de- 

 ihictiug afterwards the pressure on the opposite side of the piston. 

 He then says that the coefficient ought to be applied to the tfftctivc, 

 and not to the /oliil pressure of the steam, in which case the coefficient 

 must necessarily be smaller ; but he states that " the calculation comes 

 to the same either wav, provided a suitable coefficient be used." One 

 example will prove, that the calculation eamiot come to the same 

 either way, when the same coefficient is used in two cases in wdiich 

 the /o/al pressure is different, but the pressure on the opposite side of 

 the piston the same. 



Taking the author's example of a high pressure engine working at 

 the /otal pressure of G5 lbs. per square inch, and apjdying the coeffi- 

 cient -o, and deducting 15 from tlie product, we find 24 lbs, for the 

 practical effort applied by the engine, the effective pressure or the- 

 oretic effort being :'>(! lbs., so that if we had to pass from the latter to 

 the former, we ought to make use of the coefficient -48. (M. de Pam- 

 bour calls it -5.) If now we take another case, in which the total pres- 

 sure is only 45 lbs., we shall find the. practical effort by Tredgold's rule 

 to be 12 lbs., the theoretic effort being 30 lbs., so that 'the coefficient in 

 this case would be only -4 to make the calculation come to the same 

 as by Tredgold's rule. Thus, no suitable coefficient can be found, 

 which may be applied to the effective pressure, when the total pres- 

 sure varies, without essentially changing the rule. 



We think the author must be in error with regard to the following 

 rule, which he states (page 9J to have been used to calculate the eva- 

 poration necessary to produce a given eft'ect. 



The rule consiste'd in calculating the volume described by the piston, and 

 ill supposing that volume to have been filled with steam at the same pressure 

 as in the boiler, and then applying to it a constant coefficient. That deter- 

 mined in the iireeeding problem was usually employed, lint it was applied as 

 a divisor, Avith a view to augment the evaporation in i)roportio!i to the losses 

 represented by that coefficient. 



This rule' is doul)tless entirely destitute of foundation, nor have 

 we found it laid down by any author who has written on the steam- 

 engine. Tredgold very" pro]ierly omitted the co-efficient in the 

 divisor, which has in reality nothing at all to do w ith the question, 

 and Farey has erred liy using too high a constant divisor in the ex- 

 pression of the quantity of water evajiorated, which thus diU'ers, still 

 more widely than Tredgold's, from that condemned by M. de Pambour. 

 Sect. II. — Objections ageiiiisl that Mode of Ceilcidation. 

 These objections are in substance as follows : — The ordinary calcu- 

 lation supposes that the steam, generated in the boiler under any 

 given pressure, loses a certain constant proportion of th.it pressure 

 during its passage through the steam pipes and valves. 



That the steam having arrived in the cylinder as a pressure rather 

 less than that in the boiler, a certain fixed proportion of its power is 

 expended in friction and other resistances in the engine itself ; the 

 remainder, besides producing the useful effect, being supposed to 

 overcome the resistance on the ojiposite side of the piston; except- 

 ing, in the case of high pressure engines, the force, over and above 

 tlie pressure of the atmosphero, required to expel the steam from 

 the cylinder after having accomplished the stroke. 

 To this is objected : — 



1st. That the friction and other losses, to which tlie diminution of 

 effect is attributed, cannot absorb so great a portion of the force ot 

 the steam as is supposed. To prove this, the author shows, by ap- 

 jilying his own co-efficient -5 to the theoretic ettect of a high pressure 

 engine having an useful effect of KJU horses power, that I'redgold 

 allows a power of 12 horses to move the machinery, and of 4t) to 

 draw the piston. But, if he luul applied Tredgold's own rule, he 

 would have foiuid 1(3-79 and 54-17 instead of the above numbers, 

 which shew the absurdity of the rule in a still stronger light. 



It is however well known that there is not actually so great a loss 

 of effect in engines as the above rule attempts to account for, so that 

 it is unnecessary to make such exaggerated allowances for friction ; 

 otherwise another strong objection might be urged against the rule, 

 namely, that it gives 25lhs. total, or lOlbs. effective pressure as a limit 

 below which steam cannot be used in high pressure engines, even 

 without doing any work ; wdiich is contrary to experience. 



The second and third objections are, that the co-efficient used to 

 pass from the theoretical to the practical effect of an engine, is some- 

 times too high and sometimes too low to make the calculated results 

 harmonize w ith practice, the ratio of the effect jiroduced to the theo- 

 retical ert'ect falling in some cases as low as -25, and rising in others 

 to -8. However, until more conclusive experiments have been made, 

 we must continue to doubt the accuracy of the facts themselves. 

 As the author here instances five experiments from Wood's Trea- 

 tise on Railroads, it is necessary to say a few words respecting 

 them. The jiovver of the engines, which were stationary, was calcu- 

 lated by multiplying the area of the pistons, by the pressure of steam 

 in the boiler, and by the velocity of the pistons ; and the work done 

 was estimated by adding together all the various resistances calculated 

 by certain rules, previously determined by ex[)eriment, and multiply- 

 ing the sum by the velocity of tlie load. The ratio of the work done 

 to the power developed by the engines, calculated as above, was 

 found in the several eases : 



•256, -288, -309, -27 and -3. 



The two first experiments were made with condensing, and the 

 three latter with high pressure engines, all stationary. To account 

 for the loss of ert'ect in the former, we have the loss of pressure ex- 

 perienced by the steam during its passage from the boiler to the 

 cylinder, the pressure in the condenser and the friction of the 

 engines. 



The first experiment was made with an engine constructed by 

 Boulton and Watt, with two thirty inch cylinders, length of stroke 5 

 feet. The steam is stated to have been generated under a pressure of 

 451bs. per square inch above the atmospheric pressure, for want of 

 knowing which we must content ourselves with assuming it at 14'71bs., 

 and the (iressure in the condenser, for the same reason, at lib. per 

 square inch. 



A train of seven loaded carriages, each weighing 940Slbs., was 

 drawn up an inclined plane 2046 feet in length, and rising 154 feet 6 

 inches, in 520 seconds, the engine making 374 single strokes. 



Mr. Wood calculates the resistance of the load to have been equal 

 to 49911bs., which would require a pressure of 7t)G2-131bs. on the pis- 

 ton, or 5-020lbs. per square inch. "To this we have to add the friction 

 and losses in the engine. 



Calculating the friction of the engine by Pambour's rule, page 172 

 of the work under review, and ;iddiiig lib. for the pressure in the con- 

 denser, we find. 



Pressure corresponding to the useful clfect 5 0201bs' 



Pressure in the condenser I'OOO 



I'riclion of the engine without load ... . 1000 



Friction ow ing to the load of 3 020lbs. .... 0717 



. 2-717 



Total friction and resistance in the engine .... 



Total pressure per sc|uare inch of the piston necessary to 

 overcome all the friction, and resistance of the engine and 

 its load 7-737 



But the pressure in the boiler was 19-21bs.| therefore it would in 

 this case be necessary to adnut that the steam had lost ll-363lbs. in 

 its passage from the boiler to the cylinder, wdiich we think inadmissi- 

 ble, when the velocity of the piston was no more than 181 feet per 

 minute. We are of opinion, that the fraction -ItJSlb. would be suffi- 

 cient allowance for loss in the steam pipes and passages, in which 

 case there would still remain 1 libs, to be accounted for. This cannot 

 aU be attributed to friction and losses in the engine; but, if we add 

 seven eighths of it to the load, and the remaining eighth to the friction 

 of the engine, due to that additional load by Pambour's rule, we shall 

 have 

 Pressure corresponding to the useful effect .... 14'6451hs. 



Pressure in tlie condenser I'OOO 



Friction of th;' engine without load .... I'OOO 

 Friction due to the load 14-6451bs 2-092 



Total friction and resistance in the engine .... 4092 



I'otal pressure on each square inch of the pistons necessary to 

 overcome all the resistance and friction of the engine and its 

 load IS-TS? 



Loss of pressure in the pipes and passages • • • . 4b3 



Total pressure in the boiler ....... 19-200 



