will be the same force divided over the piston per unit of surface. Adding to it, therefore, the atmospheric pressure per unit of surface, which we shall represent by p, we shall finally have, for the total pressure, owing to the resistance, D R= (F+8M+ n M). ·FP. d2 1 In this equation M is the weight of the load in tons, n is equal to 8 lbs. and 8 1 lb. D, land dare expressed in inches, F and in pounds; thus the value of R, when found, is the pressure resulting on the piston, in pounds per square inch. The quantities D, and d might also be expressed in feet, and p in lbs. per square foot. In that case, the value of R, when found, will be the pressure per square foot on the piston. This way of expressing the resistance comes exactly to the same as the preceding one, and is sometimes more convenient for calculation. Applying this to a load of 100 t., drawn by an engine with cylinders of 11 in. diameter, stroke 16 in., wheel 5 ft., friction 110 lbs., we have, ARTICLE III. OF THE PRESSURE IN THE CYLINDER. The resistance on the piston being known, we may deduce from it the pressure of the steam, at the instant it acts as a moving power in the cylinder. It is sufficient for that to observe what passes during the motion. In regard to certain points of the engine, which, like the piston for instance, must necessarily vary in velocity during their oscillations, the uniformity of which we are speaking, consists in an exact periodical motion, which causes the velocity at each point of an oscillation to be precisely the same as it was at the same point of the preceding one. The result of this is, that if we take the duration of one of these oscillations as the unit of time, the motion will be strictly uniform. The steam, being at first shut up in the boiler at any degree of pressure, passes into the steam-pipes and from thence into the cylinders. When it arrives in those cylAs soon as the motion has acquired uniinders, the area of which is about ten times formity, which always takes place after a as great as that of the pipes, the steam must very short time and which is the regular necessarily expand and lose in the same state of the engine while travelling, the proportion of its elastic force. However, moving power, which at the beginning o the piston is still immovable; so that the the motion, was obliged to make an effort steam continuing to arrive rapidly, the necessarily greater than the resistance, equilibrium of pressure is quickly estab-needs at present only to expend a force lished between the boiler and the cylinder. just sufficient to keep the resistance in The pressure then becomes the same in equilibrium. For, if the moving power the two vessels, and the piston being im- were to apply a greater or smaller force, pelled by the force of the steam, begins the motion would be either accelerated or slowly to move. The motion is communi- retarded, whilst, in fact, it is uniform. cated to the engine and to its whole train, From that moment, consequently, the and the mass gets a certain speed. This pressure of the steam in the cylinder, which acquired speed continuing a little longer is the effort applied by the moving power, than the cause which produced it, the consequence is, that, at the following stroke, the steam finds the piston already slowly driven in a retrogade direction, at the moment when it gives it a fresh impulse, which in its turn is communicated to the total mass, where it continues to accumulate. Thus, receiving at each stroke a fresh impulse, while it still keeps the preceding one, the piston accelerates, by degrees, its speed, and the train finally acquires all the velocity the engine is able to communicate to it. We have said that, at the beginning o. 3.1416 × 60in. — 188.50 in. cirumference the motion, an equilibrium of pressure is of the wheel established between the boiler and the cylinexpressedin der; but, in proportion as the velocity of the piston increases, this piston recedes, in 100 × 8lbs. = 800 lbs. resistance of the train in lbs. 110 lbs. friction of the en gine without load. 1010 lbs. total resistance 2 x 16 in. = 32 in. inches. double or ratio of the and of the piston. Thus 1010lbs.+5.8876,946 lbs. ressistance produced on the piston. a way, before the steam, without giving it Nevertheless, the increase of the veloc a must be equal to the pressure of the resistance against the piston, which is the effort made by the resistance. This principle has been already demonstrated less extensively in another place. We know thus the pressure at which the steam is expended by the cylinder, and as we also know the volume of the cylinder, we shall be able from both to conclude the absolute expense of power which takes place at each stroke of the piston. It is that expense, which, compared with the total mass of steam of which the engine can dispose, will give us, without any difficulty, the means of determing the velocity of the motion. ARTICLE IV. OF THE EVAPORATING POWER OF THE ENGINES. er of the Engines. ment of the question, viz. the evaporating We have yet to determine the chief elepower of the engines or the quantity of water they are able to transform into steam, under a determined pressure, in a given time. ity and the diminution of the pressure have § 1. Experiments on the Evaporating Pow- With that view we undertook a series of experiments on the quantity of water evaporated by the engines of the Liverpool and Manchester Railway, during their journey from one of those towns to the other. All the tenders on that Railway having exactly the same dimensions and an uniform shape, one of them was weighed, first empty and then loaded, whereby was ascertained that every inch of water in the tank corresponded exactly with a weight of 206.5 = 31.2 lbs. resist- Then we proceeded in the following manner : We first ascertained, by means of the glass tube, at what height the water stood in the boiler at the moment of starting and then we also measured the exact height of the water in the tender. At the end of the ling. We shall explain hereafter the col-der which the steam was generated in each journey, or at the intermediate station, if umn containing the total rising of the valve, experiment. Water not being able to evapthe engine stopped to take in fresh water, which would permit all the steam generated orate under a high pressure, unless by means we first filled the boiler to the same height in the boiler to escape. of a higher temperature, we have reason to where it stood before setting off, and then In those experiments, we have mentioned suppose that, cæteris paribus, the engine we measured the water remaining in the the state of temperature of the water in must be able to evaporate less water under tender. The difference between the height the tender, because that circumstance must a more considerable pressure. But as we in the tender gave the consumption of wa- more or less facilitate the generation of shall see below, in a table we shall give on ter during the journey. steam, as it is easier to bring to the boiling the volume and temperature of the steam, When describing these experiments, in point water already warm than cold water. that between the degrees of pressure at order that the reader may see at once be- However, as the temperature we mark in which the engines constantly work, viz. fore him all the elements that have any im- the tender, exists only at the moment of between 50 and 60 lbs. effective pressure portance in the question, we shall give the starting, and as it can remain thus only per square inch, the difference of temperaload of the engine, the time it took to com- during a very small part of the journey, ture is only nine degrees by the thermomplete the journey, which shows the velocity, which lasts an hour and a half to two hours, eter, or 4 degrees difference for the mean the distance being 294 miles, the state of it really has but a very inconsiderable in- pressure, we shall easily be convinced that the spring-balance from which the pressure fluence on the result, of which the above the influence of the pressure on the quanresults, and finally the temperature of the experiments are, besides, sufficient proof.tity of water evaporated must be almost water in the tender at the moment of We have also set down the pressure un-imperceptible. Besides, when we employ a less elevated pressure, the steam generated under that pressure occupies more space, the boiler is too small to contain it, and the valve is consequently more subject to blow. The result is, that the engineman accustomed to regulate himself by the valve, seeing it continually blow, does not animate his fire so much as in the case where the valve is fixed at a higher presThe circumstance, therefore compensates for the former one, and frequently surpasses it. EXPERIMENTS ON THE EVAPORATING POWER OF THE ENGINES. Velocity Evapo- hour. per Heating surface. ration Exposed Exposed engine per hour to the in miles in each action of action of experi- Average start miles. cubic ft. sq. feet. sq. feet. 8.99 40.25 57.06 217.88 10.67 52.03 32.87 307.38 - 27.23 61.00 46.00 256.08 . 17.70 58.97 43.91 362.60 lbs. July 22 VULCAN 39.07 4646 74.34 1.17' 3' 31...32.5 5 54.5 just lukewarm July 23 ATLAS 3.17 15 50...50.7 4 53.7 cold 127.64 5937 94.99 1.58 0 50...50.1 4 cold II III. Aug. 4 37.51 5317 85.07 1.201 3 26.5...28.5 5 VESTA Aug. 15 sure. We see, consequently, in the related experiments, that the speed is the only thing that has a constant and perceptible effect on the generation of steam. The cause of this effect of the speed is, that in those engines the steam, in issuing from the cylinders, is conducted to the chimney, where it creates an artificial current of air, and acts exactly in the same manner as the bellows in giving activity to the fire. Every jet of steam represents a stroke of the bellows; and it is consequently clear, that the more rapid the motion of the engine, the more cylinders of steam will be thrown into the chimney in a minute, and the more violently also will the fire be excited. By examining the experiments, we find, in fact, that the greater the velocity of the motion, the more considerable was the evap oration; and for that reason it is necessary, in endeavoring to determine the evaporating power of the engines, to take them at their average velocity. The speed of 18 miles per hour, which is the average speed of our experiments, fulfils tolerably well that condition for the Liverpool engines. We must, therefore, consider the corresponding evaporation, which was equal to 55.82 cubic feet per hour, as the average evaporation of the engines employed. Nevertheless, we see that some of those engines have evaporated 60 or 62 cubic feet of water per hour, which makes a cubic foot per minute, or a pound of water per second. §2. Of the evaporating Power per unit of heating Surface. However, as the different engines that figured in the experiments differed in regard to their heating surface, we can determine precisely the evaporating power only, by comparing the effects of evaporation with duced. The same for the others. the dimensions of the surface which pro- ||spring-balance, and the point at which it determined these points in a positive manduces them. rose by blowing. The interval between ner, it now becomes possible, with the eleThat is the object of the two last col-these two degrees gives the rising of the ments we have at our disposal, to appre, umns we added to the preceding table, valve that took place during the experi- ciate the quantity of steam that escaped which repeat the dimensions of the heating ment, to which rising was owing the esca-during the above-mentioned experiments. surfaces of the engines, so as they were ping of the steam. Thus, in the first ex- It is sufficient for that to compare the two given with more particulars in our first periment, the valve of the VULCAN fixed columns, one of which shows the rising chapter (Chap. I. Art. II. § 3.) at 31 as point of departure, rose to 32 by that really took place, and the other the By the mean results of those two col- the blowing; consequently, the rising of rising that would have been sufficient for umns we see that the average evaporation the valve was of 14 degree on the balance. the evacuation of all the steam. By that of 55.82 cubic feet of water, was produced means, we shall find that the average by a heating surface consisting of 43.12 In the following column we have given rising that took place during the experisquare feet exposed to the action of the the quantity of rising sufficient for the ments was 12 on 46.5. A quarter of the radiating caloric, and 288.35 square feet valve of each of the engines to give issue steam was, therefore, lost through the exposed to the communicative heat. This to all the steam the engines are able to valve-we might even say a little more, is, therefore, the extent of evaporating sur- generate. This point may have been al- particularly considering that the FIREFLY face to which we must refer the effect pro-ready observed in our experiments on the engine was then in a bad condition-and pressure. We have seen, that whatever lost some of its steam through the leaks of If we admit, in consequence of the excare be taken to animate the fire, the valve||its boiler. periment related in our first chapter (Chap. can never rise beyond a certain point, be- On the other hand, that loss of steam is I. Art. II. § 3,) that each unit of surface cause then it gives issue to all the steam not accidental, but inherent to the construcexposed to the communicative heat produ-point was easy to acquire by observation the experiments on the velocity that we generated. An exact knowledge of this tion itself of those engines; and among all ces the third part of the effect that same in the numerous experiments on the velocity shall relate below, there will scarcely be surface would produce if exposed to the of the engines we are going to relate. radiating caloric, the heating surface above found a single instance in which that effect may be represented by 139.24 square feet Thus we found, for instance, that the was not produced; and when it was not, exposed to the immediate or radiating acATLAS engine, travelling at its greatest the reason was that the fire was not excited tion of the fire; and as those 139.24 speed, and stopped all of a sudden, at the to the highest possible degree. It is theresquare feet have produced in an hour the instant it was generating the most steam, fore necessary to establish a distinction ration of 55.32 cubic feet, we see that each raised its valve from 50 degress to 54 de-between the evaporating power of the ensquare foot has evaporated a volume of grees; and that the passage resulting from gine, and what we shall call their effective water expressed by 0.401 cubic foot. that rising was sufficient to evacuate all evaporating power; that is to say, the part the steam. In the same manner, the of that power which is really applied to LEEDS, VULCAN, and FURY, raised in sim-the working of the engine. ilar circumstances their valve from 31 to From the experiments above, we find 36, VESTA from 20 to 234, and FIREFLY that of all the steam generated in the boilfrom 17 to 20; the second valve of these er, three quarters only enter into the cylengines being, besides, at the points indi-inders. Thus, the evaporating power per square cated for them in our experiments on presThese degrees of rising naturally foot of heating surface exposed to the radepend, 1st, on the quantity of steam gen- diating caloric, having been found to be the valve; 2nd, on the dimensions of the the effective evaporating power expressed in erated by each engine, and the diameter of 0.401 cubic foot, the available part of it, or levers and the size of the divisions of the cubic feet of water evaporated in an hour per balance, which makes a degree by the bal- square foot of surface, is 0.301 cubic foot, or § 3. Of the effective evaporating Power ofance correspond with a greater or a lesser of a cubic foot. the Engines. evapo Thus at a velocity of 18 miles per hour, which is nearly the average speed of the engines, each square foot of heating surface, exposed to the radiating action of the fire, evaporates in an hour a volume of water of 0.401, or a little more than of a cubic foot. This is the expression of the evaporating power of the engines per unit of heating surface. Multiplying this, for each engine, by the extent of the heating surface, we shall find the total evaporating power of the engine. sure. rising; and, lastly, on the second valve, These circumstances explain the differ- 10 Finally, returning to the mean of the above experiments, the evaporating power was in each hour 55.82 cubic feet; consequently, the effective evaporating power, taken as an average for all the engines, is 41.87 cubic feet. ARTICLE V. AND THEIR CORRESPONDING EFFECTS. We must however remark, that although all that water is transformed into steam, there is only a part of it applied to the working of the engine. To be convinced of this fact, we need only examine the valves of the engines while working. We see them constantly emit a considerable ions of the balance of that engine being quantity of steam, which, instead of enter very great, on account of the proportions OF THE PROPORTIONS OF THE ENGINES, ing the cylinders, escapes immediately into of the lever, four divisions of the balance the atmosphere. This loss is a defect which are equal to a considerable rising of the it would perhaps not be difficult to correct, valve, which was sufficient to evacuate the and, if corrected, would tend considerably steam. In the LEEDS, VULCAN, and FURY, towards an economy of fuel. the second valve did not rise in the presThe plan in contemplation on the Liver-sures we employed; but the first valve, pool Railway of replacing the present cyl which is the one we consider here, was 3 inders of the engines by others of a greater in. in diameter. In the VESTA, the second diameter, will at least have that advantage, valve gave issue to the steam as well as that in case of considerable loads, it will the first; the consequence of which was, render available all the steam generated in the boiler. In the experiments of which we have given an account, not only is that loss perceptible, but it is even susceptible of being to a certain degree appreciated. that a rising of 3.5 degrees of the balance S1. Analytical expression of the Velocity of the Engine with a given Load. With these elements it is easy to determine the velocity which an engine is able to acquire with a given load. Supposing, for instance, we have a load of 100 t., tender included, attached to an engine with cylinders of 11 in. diameter, stroke 16 in., wheels 5 ft., friction 110 lbs., effective pressure of the steam in the boiler 50 lbs. per square inch, and finally effective evaporating power, such as we have found it for the average of the Liverpool engines, that is to say, 41.87 cubic feet of Under the head "State of the Spring-ran out into the fire-place, where it evapo-water evaporated in an hour. balance," we have inscribed in the table rated, and a very small quantity of steam We have already seen above (Chap. V. above according to the observation in each was really collected in the boiler. Art. II.,) that the total resistance which experiment, the point of departure of the Repeated experiments having, therefore-that load opposed to the motion of the pis |