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92 THE CIVIL ENGINEER ANI) ARCHITECT’S JOURNAL. [March, ON THE GENERAL THEORY OF THE STEAM ENGINE. Notwithstanding the number of years which have elapsed since the invention of the Steam Engine, and the immense extent to which it is now employed, as well as the great importance at present attached (particularly in some of its applications) to the perfection and economy of its working: yet our knowledge of the general theory of its action is still hut very limited. We were led to investigate this subject more fully on the perusal of a pamphlet which appeared in the course of last year, entitled, “ A New Theory of the Steam Engine,” by the Ghevaler G. de Pambour, and purporting to be an analysis of a memoir by the same author, which was read at the Institute Royale of France, during the year 1837. The author, after shewing the inaccuracy of the ordinary mode of calculation used to determine the ell'ects and proportions of steam-engines, exposes w hat he calls his new 7 theory, by means of which he undertakes to solve all problems relating to those effects and proportions. M. de Pambour’s theory consists essentially in the following laws:— 1. That the pressure in the cylinder is strictly regulated by the resistance on the piston and by nothing else. 2. That the velocity of an engine is determined by the quantity of water which can be evaporated in the boiler in a given time, and 3. That the pressure in the boiler is indifferent, provided it be at least equal to the pressure in the cylinder. The last of these laws is of very little consequence, since it can have no effect on any of the calculations; but the two former serve to solve all the problems relating to steam-engines; for, having as certained the quantity of steam generated in the boiler, and trans mitted to the cylinder, as well as the velocity of the piston, which gives the volume occupied by that steam, we find its density and elastic force, and consequently the resistance on the piston. Inversely, either of the two other quantities might be determined, the rest being given. The principal difference between M. de Pambour’s and the ordinary mode of calculation is that, according to the former, the effect of an engine is measured by the quantity of steam generated in the boiler: and according to the latter, by the quantity used in the cylinder: both which quantities must be equal, if correctly measured; unless, of course, there be any discharge through the safety valve, in which case M. de Pambour’s method w'ould fail, if he had not the means of measuring or estimating the quantity so lost; in place of which he assumes (for locomotive engines) an average loss of one fourth of the whole of the steam generated, and therefore considers the effective evaporating power of a boiler to be three fourths of its total evaporating power. Now this clearly cannot be true for all locomotive engines, nor even for one engine on all occasions ; the safety valve will be more or less open, according to the load of the engine and the pressure in the boiler; and the discharge of steam through the valve, when open to the same degree, will depend on the treasure in the boiler. The latter circumstance would, however, lave but an imperceptible influence if the difference of pressure were not very considerable, the velocity of efllux being directly as the effective pressure, and inversely as the density of the steam. The manner in which M. de Pambour arrived at the average loss of steam through the valve, show's that it is seldom, if ever, correct. He first.ascertained what rise of the valve was necessary for the dis charge of all the steam generated, which corresponds to different numbers of degrees of the spring balance for different boilers, and then observed the actual rise in a certain number of experiments made with different engines; he then compared the sum of all those rises with the sum of all the rises necessary to give egress to all the steam generated in the various boilers, taken once for each experi ment. In eleven experiments, the sum of actual rises was 12 degrees of the various spring balances, and the sum of rises necessary to give egress to all the steam, in each case 4(5-5 degrees : namely, five experiments, with engines requiring 5 degrees, give 25 ; three expe riments, with one requiring 4, give 12; two experiments, with one requiring 3, give (5; and one experiment, with an engine requiring 3'5, gives 3-5; which numbers, when added together, will be found to make up 46'5. The ratio of 12 to 46'5 being very near one-fourth, this has been taken as the average rise of the valve, and conse quently as the average loss of steam. Thus w'e see that we can make no use of the constant coefficient 075 in determining the effective evaporating power of a boiler; for it might very well happen that only one-eighth should escape through the valve, in which case we should find the effect one- seventh too little ; and if in any case there should be a loss of one- half of the steam, then we should arrive at a result 50 per cent, too great. But, setting aside the error w'e should commit by taking the average rise of the valve, how'are the following facts to be accounted for? In the table of experiments at page 229 of M. de Pambour’s “Treatise on Locomotive Engines,” we see that the Fury ascended the Sutton inclined plane, with a load equivalent to 183 tons on a level, at a speed of 13 - 33 miles an hour, the rise of the valve being 5 degrees of the spring balance, which is sufficient (see the table at page 175) to allow of the escape of all the steam generated in the boiler. We also find (page 232) that the Vesta ascended the same inclined plane, with a load equivalent to the former, at a velocity of 3'25 miles an hour, when the rise of the valve was equal to 3'5 degrees of the spring balance, or sufficient, according to the above- mentioned table, to give issue to all the steam. Also, (see page 234), the Vulcan is stated to have ascended the same inclined plane, with a load equivalent to 188 tons on a level, at a velocity of 11'42 miles an hour, the safety-valve being sufficiently open to allow of the escape of all the steam; and, in the same page, the same engine is stated to have ascended the W his ton inclined plane on another occasion, with a load equivalent to 186 tons on a level, at a speed of 1875 miles an hour, the safety-valve being open to the same degree as in the preceding case. Supposing the observations to have been correctly made, we can only account for these apparent anomalies by supposing the evapo ration to have been more rapid in the cases quoted than during the experiments which had been made, with the view of determining the rise of the valve necessary for the discharge of all the steam gene rated; for it is the absolute quantity discharged, and not the pro portion, which is determined by the size of the aperture. It is, therefore, exceedingly difficult to deduce the power developed by an engine from the evaporating power of the boiler, whenever there is any escape through the safety-valve. Before leaving the subject of these experiments, we must observe that, from their nature, they were not susceptible of that precision which is necessary to allow of their results being made the basis of accurate calculation: for, on account of the irregularities of. the road, the circumstances were continually varying, and the momentum of the trains rendered the effect of those variations less perceptible than it ought to have been. It is an essential part of the Chevalier de Pambour’s theory, that the pressure in the cylinder is independent of the pressure in the boiler, and depends solely on the resistance to be overcome. This is, to a certain extent, true; but we cannot allow that the pressure in the boiler is altogether independent of the pressure in the cylinder, for it cannot be denied that the law of the flowing of elastic fluids must obtain in this, as well as in all other cases. The pressure of the steam in the cylinder being, therefore, equal to the resistance on the opposite side of the piston, and the velocity of the piston being determined by the effective evaporating power of the boiler, which we readily allow, the pressure in the boiler will necessarily be such as to cause a corresponding efflux of steam from the boiler into the steam pipe, and through that into the cylinder. If at any moment this were not the case, the pressure would immediately begin to adjust itself, and would finally remain fixed as soon as it had arrived at that point: suppose, for example, that the pressure in the boiler is too low to cause an efflux at a sufficient velocity to supply the cylinder; the pressure will instantly rise until it be sufficient to cause an efflux at the velocity which will then be required, which is less than before, as the density is greater. This circumstance has been entirely overlooked, or rather neglected, by M. de Fambour, as well as the effect of velocity on the resistance of the air to the trains, which must considerably affect the results of his experiments. Before entering upon the general discussion of the theory, when we shall have occasion to revert to M. de Pambour’s works, we shall briefly advert to a paper on the application of steam as a moving power, which was published in the second volume of the “ Trans actions of the Institution of Civil Engineers.” We should not have stopped to notice this paper, but for the medium through which it has been brought before the public, which naturally gives a certain degree of importance to everything therein published, as it must first obtain the sanction of a body of men, who, from their profession, and the eminence which some of its members have attained in that profession, have necessarily considerable influence over the opinions of those who are personally unacquainted with mechanics. The author of the paper in question appears to have had less in view the advancement of science, as the title seems to promise, than to create a doubt in the public mind as to the correctness of the official reports of the duty performed by the Cornish engines ; anil he appears to have persuaded himself that he has demonstrated, on scientific principles, that those engines could not have performed anything like what they are reported to have done, and, consequently, that the reports are erroneous; but we hope to be able to show, satisfactorily, that he is labouring under a delusion. We will, for the sake of argument, allow that the combustion of 71bs, of coal is required to convert one cubic foot of water into steam,