part to about the middle of the former vessel.• In the pipe that goes to the reservoir is a stopcock near the top of the vessel, and two more in the two descending pipes, which latter may be placed so as to be closed by one plug traversing the two; to these cocks are fixed cranks connected together by a bar worked by the steam engine, and the passages of the cocks are so arranged that when the two lower ones in the descending pipes are closed, that in the upper one leading to the reservoir will be open, and vice versa. By this arrangement of the pipes the end of the one that passes from the top of the upper vessel to the middle of the lower one will be sometimes above the water and sometimes beneath it, according as its level varies in this vessel; when the first happens the patentee states that the steam will rise up through into the upper vessel, and will force the water which this contains through the other descending pipe into the lower vessel; and when the latter case occurs, and its lower end is immersed, then the passage of the water will be interrupted. It is mentioned that this latter apparatus may be used with any other sort of steam-boiler as well as those described. 231. Finally, a method is recited for freeing the tubes of the boilers from the sediment and incrustations that are more or less formed in all boilers, which method consists in filling them, if formed of iron, with a mixture of one part of muriatic acid to 100 parts of water, or with a mixture of vinegar and water, in the proportion of a pint of the former to three gallons of the latter; and if the tubes are made of copper, then sulphuric acid is directed to be added to marine salt, in the proportion of half an ounce to a pound, and with a proper quantity of water to be put into them. These mixtures are to be used cold, and a little while after one of them is poured in, a small fire of chips is to be made under the boiler, to make the fluid circulate and bring fresh portions of it in contact with the concretions; and, when these are observed to be sufficiently dissolved, the liquor is to be run off through a cock in the bottom of the boiler along with the undissolved siliceous matter. 232. Mr. M'Curdy's apparatus has excited too much attention to be passed without particular notice. It consists of eleven cylinders, twelve feet long and twelve inches in diameter, placed horizontally in two rows, like gas retorts, in a furnace, six being in the lowest row and five in the upper one over the intervals of these, and a fire-place being made under each three of the cylinders in the lower row. Each cylinder contains another cylinder within it, so much smaller as to leave a space between them of about an inch in extent, which space is to be maintained by bands, or pins, reaching so much beyond the inner cylinders, and placed on them at regular distances asunder. 233. Above the eleven double cylinders, or steam-generators, in the same furnace, a twelfth cylinder of the same length, but somewhat more than thirty inches in diameter, is placed, which Mr. M'Curdy calls a steumometer, and which will hold ten times the contents of all the spaces between the inner and outer cylinders; the eleven outer cylinders are connected by tubes, which run from one to the other sideways near their tops in each row, and from the lower to the upper row; a tube also passes from the upper row of cylinders to the steamometer, and another goes from the latter to the working cylinder of a steam engine. The supply of water is given by a forcing pump, which draws it from the reservoir and impels it into the space between the inner and outer cylinders, which are placed at the outside of the lowest row, and thence through the tubes and the spaces between all the other inner and outer cylinders up to the steamometer; at the bottom of which latter a cock is placed, by which the ascent of the water to that point may be known; while by another cock, fixed between the forcing pump and the reservoir, the degree of the supply of water is regulated. A safety valve on the steam tube, close to where it passes from the steamometer on its way to the steam engine, completes the apparatus. 234. In pursuing the history of the steam engine, we have already examined the most perfect form of the single-acting apparatus; but an engine of double powers was soon produced by the comprehensive mind of Mr. Watt. He had introduced steam acting against a piston to press it downwards; he now formed a communication between both sides of the piston and the boiler, and also with the condenser, and made the steam act to press the piston upwards as well as downwards. 235. The mechanism was now, as far as the principle went, perfect; and it was freed, for the first time, from the enormous dead weight of counterpoises, which had hung on it from the first attempts of Newcomen; and the equally enormous load which was used in the construction of the various parts, for the purpose of equalising the motion. 236. The cylinder a, plate VII., is enclosed in a jacket or casing like the single engine, having a similar interval, which may be filled with steam or air. The piston b is attached to the lever-beam by the rod r. 1, 2, 3, 4, are the valves which admit steam to the cylinder, or open a communication between the upper and under sides of the piston and the condenser. g is the pipe leading from the valves to the condenser. m, m, the levers or spanners, which are elevated or depressed by the tappets or pins n, z, in the plug-frame, and open or shut the valves to which they may be connected. is the condenser; L, a pipe connecting it with air-pump i, and a second air-pump E. C, the piston-rod of this second pump, attached like the other, . to the lever-beam. F, a pipe from the cold water-pump q, to supply the reservoir in which the condenser and its pumps are placed. k, a trough or reservoir into which the water heated by the condensation of steam in the condenser, which is raised by the air-pump, is pumped back by м into the boiler. G, a pulley; and H, an endless chain moving over it, also going round a pulley fixed on the upright axis of the conical pendulum or governor z. R, the handle of the lever which regulates the quantity of injection water admitted. P, P, the masonry or wall on which the cylinder, and other parts of the machine are placed. d, a pipe conveying steam from the boiler to the cylinder. B, a cock or valve, called the throttle-valve or regulator, placed on the pipe conveying the steam from the boiler, and which is moved by the levers shown as supported at D, and connected with the conical pendulum. T,Q, Q, Q, w, are a system of levers, called the parallel motion. is the axis of the lever-beam y. 237. The motion at first is produced in this machine in the same manner as in the single engine,-by filling the condenser and cylinder with steam, and then opening the injection-cock. 238. This process may be considered to have been gone through, and that the piston has arrived at the top of the cylinder. At this moment the tappets n, n, and levers m, m, open the valves 1 and 4, and shut 3 and 2. The steam from the boiler now acts on the upper side of the piston, while a vacuum is formed under it by valve 4 opening a communication between that part of the cylinder beneath it and the condenser. The piston is therefore pressed by the elasticity of the steam to the bottom; when it has arrived at the lowest point, the tappets on the plug frame, which also descend with the piston-rod, shut the valves 1 and 4, and open 3 and 2. The steam from the boiler, instead of flowing in at the top of the cylinder, is admitted at the bottom, and a communication is opened between the upper end of the cylinder and the condenser: a vacuum is then produced above the piston, and the elasticity of the steam (instead of the counterpoise in our last figure) forces it upwards. When it is elevated to the required height, the tappets again act on the spanners, and prevent the further flow of steam beneath the piston, and admits it at its upper end, opening at the same moment a communication between the lower end and condensing apparatus. The motion of the piston is then reversed, and this alternation may be continued indefinitely. 239. The mode of pumping out the water from the condenser, being the same as that in the single engine, will be easily understood from an inspection of our figure. In order to show the four valves in section, a pipe placed in the same direction, and opposite to o, has been omitted in the engraving; it connects the top of the cylinder and the condenser together. 240. The power of the condensing engine is easily known by ascertaining the temperature of the steam, which moves the piston, the area of the piston, and the temperature of the vapor which remains in the condenser. We know from experiment that steam of the temperature of 212° will balance the pressure of the atmosphere, or, what is the same thing, will force a piston into a vacuum with a force equal to about fourteen pounds and three quarters weight for every square inch of the area of the piston. The difference between the elasticity of the steam in the condenser, and that issuing from the boiler, will be the measure of the power of the engine. It is however found most expedient to raise the steam to a somewhat higher temperature than 212°, so as to produce a pressure between seventeen and eighteen pounds on each square inch of the piston; yet, in practice, from the imperfect vacuum which is made in the condenser, and after making allowance for the friction of the piston on the sides of the cylinder, and for the friction of the various parts of the intermediate machinery, this pressure of eighteen pounds on each square inch of the piston, cannot raise more water per inch than would weigh about eight pounds and a half, so that somewhat more than a half of the whole power of the steam is absorbed to give motion to the intermediate mechanism. The height to which this weight can be elevated depends on the length of the cylinder, and to raise the same weight to twice the height by the same piston requires double the quantity of steam. The double condensing engine will also perform double the work of a single engine in about half the time, but it requires double the quantity of steam.. So that in all cases, under the same circumstances, the work performed will be as the quantity of steam used. 241. When the impulse of the steam impelled the piston only in one direction (downwards) its motion could be imparted to the beam by means of chains; but, when the impulse was to be communicated upwards as well as downwards, some other contrivance for connecting the beam and piston became necessary; and one of the conditions of this contrivance must be, to convert the motion in a curved path of the end of the leverbeam, into a rectilineal motion of the cylinder piston-rod. Mr. Watt, in his earlier engines, used to form the end of the beam as a sector with teeth, which worked into a rack fixed on the end of the piston-rod; this allowed the rod to move perpendicularly upwards or downwards, but it was very inelegant in appearance, worked with a great noise, and was easily deranged, especially at the instant when the direction of the motion was changed. 242. Even after the motion of the piston was equalised, by shutting off the steam sooner or later from the cylinder, another source of irregularity was found to arise from the varying quantity of steam which in different states of the fire under the boiler, was admitted into the cylinder. Several modes of adjustment occurred to Mr. Watt. The one most generally employed, and probably as accurate as any, was, by placing a valve in the pipe connecting the boiler and the cylinder, which could be made to increase or diminish the steam-way. The next improvement was to make this valve, called a throttle-valve, a self-acting one, and to admit of its being so adjusted that, when the piston was moving with too great a velocity, it would admit less steam into the cylinder, aud so diminish its speed, and on the contrary admit a greater quantity when it was moving too slow. 243. A similar irregularity in the motion of corn-mills, from the varying quantity of water or resistance, had early exercised the ingenuity of millers, to obtain some means by which its injurious effects could be obviated. One of the most usual modes was by means of a couple of heavy balls, attached by a jointed rod, which were made to revolve by being connected with. the spindle or axis of the mill-stones. the stones were moving at a great speed, the meal, by the rise of the stones, was too coarse; When and, on the contrary, when the motion was too slow, the meal produced was small in quantity, and too fine. The attached balls, which were called a lift-tenter, by their centrifugal force either raised or lowered a stage in which the arbor of the spindle revolved, and brought the mill-stones nearer, or removed them farther from each other, as they might be adjusted. This most ingenious regulator was adopted by Mr. Watt, and was applied to regulate the opening and shutting of the throttle-valve of his improved engine. 244. Mr. Hornblower's engine combined the high pressure principle, and the condensing apparatus, in one engine. We are not to consider this engine as being on a different principle from Mr. Watt's, but as applying his principles of condensation and expansion in a different manner from what Mr. Watt does. Mr. Hornblower obtained a patent in 1781 for a machine or engine for raising water by means of fire, and the specification of the patent was as follows:-First: I use two vessels, in which the steam is to act, and which in other engines are called cylinders. Secondly: I employ the steam after it has acted in the first vessel to operate a second time in the other, by permitting it to expand itself, which I do by connecting the vessels together, and forming proper channels and apertures, whereby the steam shall occasionally go in and out of the said vessels. Thirdly: I condense the steam, by causing it to pass in contact with metallime surfaces, while water is applied to the opposite side. Fourthly: To discharge the engine of the water used to condense the steam, I suspend a column of water in a tube or vessel constructed for that purpose, on the principles of the barometer, the upper end having open communication with the steam vessels, and the lower end being immersed in a vessel of water. Fifthly: To discharge the air which enters the steam-vessels with the condensing water, or otherwise, I introduce it into a separate vessel, whence it is protruded by the admission of steam. Sixthly: That the condensed vapor shall not remain in the steamvessel in which the steam is condensed, I collect it into another vessel, which has open communication with the steam-vessels, and the water in the mine, reservoir, or river. Lastly, in cases where the atmosphere is to be employed to act on the piston, I use a piston so constructed as to admit steam round its periphery, and in contact with the sides of the steam-vessel, thereby to prevent the external air from passing in between the piston and the sides of the steam-vessel.' 245. The following is a description of this engine by the inventor, as it was published in the Encyclopædia Britannica. Let A and B (plate III. fig. 1) represent two cylinders, of which A is the largest; a piston moves in each, having their rods, C and D, moving through collars at E and F. These cylinders may be supplied with steam from the boiler by means of the square pipe G, which has a flanch to connect it with the rest of the steam-pipe. This square part is represented as branching off to both cylinders: c and d are two cocks, which have handles and tumblers as usual, worked by the plug-beam W. On the fore-side of the cylinders, that is, the side next the eye, is represented another communicating pipe, whose section is also square, or rectangular, having also two cocks a, b. The pipe Y, immediately under the cock 6, establishes a communication between the upper and lower parts of the small cylinder B, by opening the cock b. There is a similar pipe on the other side of the cylinder A, immediately under the cock d. 246. When the cocks c and a are open, and the cocks 6 and d are shut, the steam from the boiler has free admission into the upper part of the small cylinder B, and the steam from the lower part of B has free admission into the upper part of the great cylinder A; but the upper part of each cylinder has no communication with its lower part. 247. From the bottom of the great cylinder proceeds the eduction-pipe K, having a valve at its opening into the cylinder; it then bends downward, and is connected with the conical condenser L. The condenser is fixed on a hollow box M, on which stand the pumps N and O, for extracting the air and water, which last runs along the trough T, into a cistern U, from which it is raised by the pump V, for recruiting the boiler, being already nearly boiling hot. Immediately under the condenser there is a spigotvalve, at S, over which is a small jet-pipe, reaching to the bend of the eduction-pipe K. The whole of the condensing apparatus is contained in a cistern, R, of cold water; a small pipe, P, comes from the side of the condenser, and terminates on the bottom of the trough T, and is there covered with a valve, Q, which is kept tight by the water that is always running over it. 248. Lastly, the pump-rods. X, cause the outer end of the beam to preponderate, so that the quiescent position of the beam is that represented in the figure, the pistons being at the top of the cylinders. 249. Suppose all the cocks open, and steam coming in copiously from the boiler, and no condensation going on in L, the steam must drive out all the air, and at last follow it through the valve Q. Now shut the cocks b and d, and open the valve S of the condenser; the condensation will immediately commence, and draw off the steam from the lower part of the great cylinder. There is now no pressure on the under side of the piston of the great cylinder A, and it immediately descends. The communication Y, between the lower part of the small cylinder B and the upper part of the great cylinder A, being open, the steam will go from the lower part of B, into the space left by the descent of the piston of A. It must therefore expand, and its elasticity inust diminish, and will no longer balance the pressure of the steam coming from the boiler, and pressing above the piston of B. 250. This piston, therefore, if not withheld by the beam, would descend till it came in equilibrio, from having steam of equal density above and below it. But it cannot descend so fast; for the cylinder A is larger than B, and the arch of the beam, at which the great piston is suspended, is no longer than the arm which supports the piston of B; therefore, when the piston of B has descended as far as the beam will permit it, the steam between the two pistons occupies a larger space than it did when both pistons were at the top of their cylinders, and its density dimin.shes as its bulk increases. The steam beneath the small piston is, therefore, not a balance for the steam on the upper side of the same, and the piston B will act to depress the beam with all the difference of these pressures. 251. The slightest view of the subject must show the reader that, as the pistons descend, the steam that is between them will grow continually rarer and less elastic, and that both pistons will draw the beam downwards. Suppose, now, that each one had reached the bottom of its cylinder, shut the cock a, and the eduction-valve at the bottom of A, and open the cocks b and d. The communication being now established between the upper and lower part of each cylinder, their pistons will be pressed equally on the upper and lower surfaces; in this situation nothing, therefore, hinders the counter-weight from raising the pistons to the top. 252. Suppose them arrived at the top: the cylinder B is at this time filled with steam of the ordinary density; and the cylinder A with an equal absolute quantity of steam, but expanded into a larger space. Shut the cocks b and d, and open the cock a, and the eduction-valve at the bottom of A; the condensation will again operate, and cause the pistons to descend; and thus the operation may be repeated as long as steam is supplied; and once full of the cylinder B, of ordinary steam, is expended during each working stroke. 253. The cocks of this engine are composed of two flat circular plates, ground very true to each other, and one of them turns round on a pin through their centres: each is pierced with three sectorial apertures, exactly corresponding with each other, and occupying a little less than onehalf of their surfaces. By turning the moveable plate so that the apertures coincide, a large passage is opened for the steam; and, by turning it so that the solid part of the one covers the aperture of the other, the cock is shut. Such regulators are now very common in the cast-iron stoves for warming rooms. Mr. Hornblower's contrivance for making the collars for the pistonrods air-tight is thus: the collar is in fact two, placed at a small distance from each other; and a small pipe, branching off from the steam-pipe, communicates with the space between the collars. This steam, being a little stronger than the pressure of the atmosphere, effectually prevents the air from penetrating through the upper collar; and, though a little steam should get through the lower collar into the cylinder A, it can do no harm. The manner of making this stuffing-box is as follows: on the top of the cylinder is a box to contain something soft, yet pretty close, to embrace the piston-rod in its motion up and down; and this is usually a sort of plaited rope of white yarn, nicely laid in, and rammed down gently, occupying about a third of its depth; upon that is placed a sort of tripod, having a flat ring of brass for its upper, and another for its lower part; and these rings are in breadth equal to the space between the piston-rod and the side of the box. This compound ring being put on over the end of the piston-rod, another quantity of this rope is to be put upon it, and gently rammed as before; then there is a hollow space left between these two packings, and that space VOL. XXI. is to be supplied with strong steam from the boiler. Thus is the packing about the piston-' rod kept in such a state as to prevent the air from entering the cylinder when at any time there may be a partial vacuum above the piston. 254. Mr. Hornblower's description of this engine was followed by a mathematical investigation of the principles of its action, by the ingenious professor Robison, which demonstrates that it is the same thing in effect as Mr. Watt's expansionengine; but, though this is true, there is a considerable difference in the steps by which the effect is attained, which gives an important advantage when it is reduced to practice. We shall give an investigation in a more popular form, using only common arithmetic. Mr. Hornblower assumed that the power or pressure of steam is inversely as the space into which the steam is expanded: this is the case with air, and, for the present, we will grant it to be so with steam, and reason from the same data as the ingenious inventor gives us. 255. To explain clearly what passes in the two cylinders, we must deviate from the precise form of the engine, and divest ourselves of one complication of ideas, by reducing both cylinders to the same stroke; therefore, suppose the engine to be made like fig. 2, which represents the two cylinders placed one upon the other, the lower one being double the capacity of the upper one, and both pistons being attached to the same rod, which may be applied to the end of the beam, so that the descent of the pistons must draw up the load at the opposite end of the beam. 256. Then, if we suppose the small piston to be ten inches in diameter, the great piston must be 14:14 inches; and to avoid all difficulties of the ratio of the expansion, and the pressure of steam, we will suppose the engine to be worked by the pressure of atmospheric air instead of steam; and, for the convenience of round numbers in our calculation, we will consider the pressure at only ten pounds per circular inch on the surface of the piston. 257. The area of the small piston will be 100 circular inches, and, being assumed to move without friction, the pressure upon it will be 10 X 100=1000 lbs. The area of the great piston is twice as much, or 200 circular inches, and the pressure 2000 lbs. 258. Suppose both pistons to be at the top of their respective cylinders; let the atmospheric air be admitted to press freely upon the upper surface of the small piston; and suppose the space between the two pistons filled with air of the same density, while there is a perfect vacuum made in the lower part of the great cylinder, beneath its piston. 259. Under these circumstances, the two pistons will begin to descend with something less than 2000 lbs. of load upon the outer end of the beam, because there are 2000 lbs. of pressure on the great piston by the air contained in the space between the two pistons, bearing on the 200 inches of surface with a weight of ten pounds per inch; and beneath this piston there is nothing to counteract the pressure. At the same time the small piston, having air of equal density above and below it, is in equilibrio. L At one-fourth of the descentthe power will have diminished, by regular decrements, to Because the air between the two pistons must occupy three-fourths of the small cylinder, and one-fourth of the great cylinder, which is a space equal to one and one-fourth of the original space which it filled; therefore the spaces will be as five to four; and, if the density of air is as the inverse proportion of the space which it occupies, the pressure on the great piston must be as four to five, or four-fifths of 2000= 1600. At one-half of the descent 1600 the power will have di-13331 minished to . Because at this position the air between the pistons occupies one-half of the small cylinder and one-half of the great one, which is a space equal to one and one-half of the space it filled originally. The spaces will therefore be as six to four, and the pressure on the great piston as four to six, or two-thirds of 2000 = 13331. At three-fourths of the de scent the power will be 11429 Because the air must now oc- At first the power will be Because the piston is in equilibrio, having 1000 lbs. pressing upwards, and 1000 lbs. downwards. {At one-fourth the power 200 At one-fourth will Because the equilibrium does not continue, and at one-fourth of the descent the pressure beneath the small piston is reduced by the expansion of the air between the two pistons to four-fifths of 1000 800 lbs., while the pressure above the piston continues to be 1000. The power is, therefore, 1000 - 800 =200. At one-half of the descent the power will have 333 At one-half increased to Because the pressure beneath is diminished by the increased rarity of the air to two-thirds of 1000 6663, while the downward pressure continues to be 1000. The power is therefore 1000-666-331. At three-fourths of the 1800 1666 At the bottom of the cylinder 1000 At the bottom the power 500 At the bottom the power will be Because the air must occupy Sum of the powers exerted will be Because the air beneath the piston is reduced to one-half of its pressure, or 500, which, deducted from 1000, leaves 500. Sum of the powers of the small piston 1500 |