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 nietalline 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 b, 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 band 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 must diminish, and will no longer balance the pressure of the steam coming from the boiler, and pressing above the piston of B. 250. This pistou, 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 diminishes 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 pistonrod 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, be neath 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 first the power will be. 2000 In consequence of the pressure of 10 lbs. per circular inch upon its upper surface, and no pressure beneath. At one-fourth of the descent the 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-13334 Because at this position the air therefore be as six to four, At three-fourths of the de- Because the air must now oc- At the bottom of the cylinder 1000 the power will be Because the air must occupy Sum of the powers exerted by the great piston in its 7076 descent At first the power will be Because the piston is in equilibrio, having 1000 lbs. pressing upwards, and 1000 lbs. downwards. |