1839.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



347 



The (luaiitity of water evaporated on the former supposition would 

 liave been no more than 0'G32 cubic feet per minute, hardly half as 

 much as would have been allowed Ijy Watt, namely, 1'341 culiic feet. 

 On the latter supposition the expenditure must have been 1-43U cubic 

 feet., or very nearly the same as Watt's allowance. It is, therefore, 

 nnich more likely that the pressure in the cylinder was lS'7371bs. per 

 square inch, than 7-7171bs., and if so, the resistances are necessarily 

 estimated too low, or the pressure in the boiler too high, or, what is 

 more probable, both these errors have been committed at once. It is 

 also more than possible that PamboiU''s rule gives too low a result for 

 the friction of the engine in this case. However it may be, we cannot 

 ]iut any confidence in such experiments, nor admit them as arguments 

 against the ordinary mode of calculation. We are mnch more ready 

 to admit that the useful ettects may sometimes amount to -6 of the 

 whole effort of the engine ; for we believe, as we have already stated, 

 that the allowance made for friction is generally much too high. 



We quote the fourth objection, as it embraces an important part of 

 the theory of the steam engine, and we have some observations to 

 make upon it, which materially affect the views explained by the 

 author. He expresses himself thus : 



4th. The measure of the theoretic effect of the engine results from tlirce 

 elements, to wit : the surface of the piston, the pressm'e of the steam, and 

 the velocity of tlie motion. Tlie canses wliich are said to explain the reduc- 

 tion to which this theoretic effect is liable, are : first, the friction of tlie 

 engine, then the contraction of the passages, their changes of direction, the 

 friction of the steam, its waste and its condensation. Now of the last five 

 causes, the condensation is the only one that can diminish the pressure of 

 the steam iluring its passage, and that condensation is almost entirely oh- 

 ■ viatcd Ijy the precautions used in practice : all the remaining causes of ve- 

 (hiction act merely on the velocity. If then these causes produce definitively 

 a reduction in the theoretic effect, it can only he by reason of their action on 

 the velocity. 



What is here objected to is, therefore, that the pressure of the 

 steam in the cylinder is supposed to be diminished by the contraction 

 of the passages, their changes of direction, the friction of the steam, 

 and its waste, which M. de Pambour asserts to act merely on the ve- 

 locity ; but this objection, as regards the contraction of the passages, 

 is cancelled in Section VII., where he states that the degree of open- 

 ing of the regulator acts upon the pressure in the boiler, but can have 

 no inflnence on the pressure in the cylinder. Now this is admitting 

 that the area of the passages influences the ratio of the two pres- 

 sures ; so that, if we suppose either of them known, the other must 

 be determined by the area of the passages. It is most natural to as- 

 sume as knswn that pressure which may be immediately measured, 

 namely, that in the boiler, and conclude from that on the pressure in 

 cylinder, which is the method usually followed. If, with the same 

 load, and consecjuently the same pressure in the cylinder, a contrac- 

 tion of the passages causes the pressure in the boiler to rise (which 

 must necessarily be the effect, if the velocity remains the same,) it is 

 very clear that, with the same pressure in the boiler, a contraction of 

 the passages will necessitate a diminution of the load, which must be 

 accompanied by a diminution of the pressure in the cylinder. Thus, 

 the contraction of the passages, and every other cause which tends to 

 retard the motion of the steam from the boiler to the cylinder, may 

 be said to diminish the pressure in the cylinder, and they cannot be 

 said to act upon the velocity, since that is known. This last objec- 

 tion, therefore, falls to the ground. 



The formulEe objected to in Section III. are based on a law, (that of 

 the velocity of falling bodies, which as there applied, have no re- 

 ference wdiatever to the velocity of the piston of an engine, which 

 they were intended to determine. We shall therefore merely re- 

 mark that the velocity sought was that corresponding to the maximum 

 mefal (ffect of an engine, and not to a given load, as M. de Pambour 

 seems to kave supposed. 



Sect. IV. — View of Ihe Theory proposed. 



This section contains only the basis of this theory, consisting of the 

 two following laws : — -Ist. That there is necessarily equilibrium be- 

 tween the pressure of the steam in the cylinder and the resistance 

 against the piston; and 2d. That there is also a necessary equality 

 between the production of steam and its expenditure. These laws 

 are undoubtedly true, and we believe the author of the work before 

 us to have been the first to point them out ; but, as it is of importance 

 that no inaccuracy, however slight, should be found in the exposition 

 of a principle, which is supposed of itself to explain the whole 

 theory of the steam engine, we shall quote the paragraph from page 

 2t1, in order to point out an error, which, though perhaps too trifling 

 to be of any consequence in practice, should nevertheless be avoided 

 in the expression of a general law. 



Now in every machine wldcli has attained a uniform motion, tlie power is 

 strictly in equililirio witli tlie resistance ; for were it greater or less, there 

 would he acceleration or retardation of motion, which is not the case. In a 

 steam-engine, tlie force applied hy the mover is no other than the pressure 

 of the steam against the piston or in ihe cylinder. This pressure then, iu 

 the cyUuder, is strictly equal to the resistance opposed hy the load against 

 the piston. 



Consequently, the steam in passing from the boiler into the cylinder 

 changes its pressure, assuming that which represents the resistance to the 

 piston. This principle, of itself, explains all the theory of the steam-engine, 

 and in a manner lays its play open. 



The error alluded to is, that " the pressure in the cylinder is strictly 

 equal to the resistance opposed by the load against the piston." 

 Now the mean resistance opposed by tlie load can never exceed the 

 pressure which the steam exerts against the piston, which, wdule the 

 piston is in motion, can never be strictly equal to its wdiole pressure 

 m the cylinder, though in most, or even in all cases which occur in 

 practice, the difference may be inappreciable. It would, however, 

 have been preferable under these circumstances, if the word practi- 

 cally had been used instead of strictly. 



The method of calculating the eflbrt applied on the pison, consists 

 in ascertaining the quantity of water evaporated and transmitted in 

 the form of steam to the cylinder in a given time, which, compared 

 with the distance travelled by the piston in that time, gives the den- 

 sity of the steam in the cylinder, whence its elastic force may be 

 deduced. By the ordinary method, the elastic force of the steam 

 is assumed to be reduced in a constant proportion during its passage 

 from the boiler to the cylinder, the loss of elasticity being supposed 

 to be very trifling w ith steam |)ipes, &c. of suitafile dimensions, and a 

 moderate velocity of the piston ; while M. de Pambour asserts that 

 that loss may be very great, even as much as one half of the total 

 pressure in the boiler. This is however in opposition to the law of 

 the flowing of elastic fluids, which must obtain in a steam engine as 

 well as under any other circumstances. Now it must be extremely 

 diflicult to ascertain with any degree of accuracy the quantity of 

 water which passes in the form of steam through the cylinder, parti- 

 cularly in locomotive engines, from experiments on which M. de 

 Pambour deduced his theory ; for the rise of the safety valve can by 

 no means be atlinitted as an accurate measure of the quantity of steam 

 escaiiing through it, until all the phenomena connected with it have 

 been more satisfactorily elucidated ; and no experiment can be satis- 

 factory, imless the engine be compelled to work for a considerable 

 length of time under precisely the same circumstances. 



.Sect. V. — J\'ew proofs of the accuracy of the theory proposed, and of 

 the inaccuracy of the ordinary theory. 



The tendency of these proofs, is to establish " that the pressure of 

 the steam in the cylinder is strictly regulated by the resistance on the 

 piston, and by nothing else," and implicitly, that the ratio of the 

 pressure in the cylinder to that in the boiler is independent of the area 

 of the steam passages and the velocity of the piston. It is assumed 

 throughout that the pressure in the boiler is, or may be, the same with 

 all loads. All this is, however, distinctly contradicted in the last 

 paragraph of the section. The following extracts will prove the truth 

 of our assertions. 



Ill fact, were it actually true that the steam he expended in the cylinder, 

 cither at the pressure of the boiler, or at any other pressure that were in 

 any fixed ratio whatever to that of the boiler, then, since the quantity of 

 steam raised per minute in the boiler would he expended Iiy the cylinder at 

 one and the same pressure in all cases, and would consequently fill the cylin- 

 der a fixed number of times in a minute, it would follow that the engine, so 

 long as it should work with the same pressure in the boiler and the same 

 apertm-es or steam passages, would assume the same velocity with .-ill loads. 

 Now, we see that the very contrary fakes place ; for, the lighter the load, the 

 greater becomes the velocity of the engine. 



The .Itlas engine, for instance, evajiorated 132 cubic feet of water in 

 drawing 19.i-5 tons, and 95 cubic feet only in drawing 127-0 tons. Since the 

 same number of eylinilers-fuU of steam was expended in each case, tlie steam 

 of the first must have been of a density different from that of the second ; 

 and here again it is manifest that, notwithstanding the equality of the pres- 

 sure in the boiler, and of the opening of the regulator in the two cases, the 

 density of the expendeil steam followed the intensity of the resistance, that 

 is to say, the pressure of steam in the cyhnder was regulated hy the 

 resistance. 



Othly. It is clear, moreover, that if the pressure in the cylinder were, as it 

 is thought, constant for a given pressure in the boiler, then after an engine 

 has lieeu found capable of drawing a certain load with a certain jiressure, 

 and of coiuniunicating to it a uniform motion, it would follow that the same 

 engine could never draw a less load with the same pressure in tlie boiler, 

 without commimicating to it a velocity indefinitely accelerated ; since the 



