I must get you to understand this clearly, so that the principle may become plain—“clear as mud,” as Paddy would say. I told you that the air pressed on every square inch of surface with a force of about 15 lbs. We do not feel it, because we are equally pressed on all sides—from within as well as from without—so that atmospheric pressure is balanced. Sometimes this is a very good thing. We should, I think, hardly like to carry about the huge weight pressing upon our shoulders, if something did not counteract it for us, so that we cannot feel it. Indeed, if it were otherwise, we should become flat as pancakes in no time—“totally chawed up.”
But sometimes we should prefer to get rid of the air altogether—and I can tell you it is not easy to do so, unless we put something into its place; and we want perhaps simply to get rid of it, and make use of the room it occupied. We require to do this in the present instance, and in fact we have just done it. If the whole space below the piston, when we begin to work, is filled with water, it is plain there can be no air below it; and when the steam has raised it, there is still no air below it, but only steam. We then apply cold to the cylinder by removing the lamp and squirting cold water against it, which has the effect of reducing the steam to water again, which will occupy 1 inch of space only. We, therefore, now have a space of 1600 cubic inches with neither air nor water in it; and so, if the piston is 1 inch in size, there will be the 15 lb. pressure of the atmosphere upon it, and nothing below to balance it, for we have formed a vacuum below it, and of course this 15 lb. weight will press it rapidly down. It did so; and we therefore were enabled to raise 15 lb. in the scale-pan. You will know, therefore, henceforth, exactly what I mean by a vacuum and atmospheric pressure. It is, you see, the latter which does the work when a vacuum is formed as above; but you can easily understand that it might be possible to use both the atmospheric pressure and the pressure of steam as well, which is done in the condensing steam-engine.
In the earliest engine, called the Atmospheric for the reason above stated, the top of the cylinder was left entirely open, as in our sketch; but the condensing water was not applied outside the cylinder, but descended from a cistern above, and formed a little jet or fountain in the bottom of the cylinder at the very moment that the piston reached its highest point. Down it, therefore, came, drawing up the pump-rod. When at the bottom the jet of water ceased. Steam was again formed below the piston, which raised it as before; and the process being repeated, the required work was done. A boy, to turn a couple of taps, to let on or off the water or steam, was all the attendance required.
For some time the atmospheric engine, the invention of Newcomen, was the only one in general use; and even this was, in those days (1705-1720), so difficult to construct that its great power was comparatively seldom resorted to, even for pumping, for which it was nevertheless admirably suited. The huge cylinder required to be accurately bored, while there were no adequate means of doing such work; and although the piston was “packed,” by being wound round with hemp, it was difficult to keep it sufficiently tight, yet at the same time to give it adequate “play.” Then, another drawback appeared, which, though of less importance in some districts, absolutely prevented the introduction of this engine into many parts of the country. The consumption of coal was enormous in proportion to the power gained. We can easily understand the reason of this, when we consider the means used for producing a vacuum in the cylinder below the piston. The water introduced for the purpose, chilled, not only the steam, but cylinder and piston also; and therefore, before a second stroke could be made, these had to be again heated to the temperature of boiling water. The coal required for the latter purpose was therefore wasted, causing a dead loss to the proprietor.
So matters continued for some time, until a mathematical instrument-maker of Glasgow, named Watt, about the year 1760, began to turn his attention to the subject; and having to repair a model of Newcomen’s engine belonging to the University of Glasgow, the idea seems to have first struck him of condensing the steam in a separate vessel, so as to avoid cooling the cylinder after each upward stroke of the piston. This was the grand secret which gave the first impetus to the use of steam-engines; and from that day to this these mighty workmen, whose muscles and sinews never become weary, have been gradually attaining perfection. Yet it may be fairly stated that the most modern form of condensing engine in use is but an improvement upon Watt’s in details of construction and accuracy of workmanship. For Watt did not stand still in his work; but after having devised a separate condenser, he further suggested the idea of closing the top of the cylinder, which had hitherto been left open to the influence of the atmosphere; and rejecting the latter as the means of giving motion to the piston, he made use of the expansive power of steam on each side of the piston alternately, while a vacuum was also alternately produced on either side of it by the condensation of the steam.
The atmospheric engine was thus wholly displaced. The saving of fuel in the working of the machine was so great, that the stipulation of the inventor, that one-third of the money so saved should be his, raised him from comparative poverty to affluence in a very short time. Watt, however, had still to contend with great difficulties in the actual construction of his engines. He was in the same “fix” as some of my young readers, who are very desirous to make some small model, but have little else than a pocket-knife and gimblet to do it with. For there were no large steam-lathes, slide-rests, planing and boring machines, procurable in those days, and even the heaviest work had to be done by hand, if indeed those can be called hand-tools which had frequently to be sat upon to keep them up to cut. It was therefore impossible for Watt to carry out his designs with anything like accuracy of workmanship, else it is probable that he would have advanced the steam-engine even further towards perfection than he did. In spite of these drawbacks, however, this great inventor lived to see his merits universally acknowledged, and to witness the actual working of very many of these wonderful and useful machines.
The first necessity which occurred from closing the cylinder at both ends was the devising some means to allow the piston-rod to pass and repass through one end without permitting the steam to escape. This was effected by a stuffing-box, which is represented in Fig. 57, A, B,—the first being a sectional drawing, which you must learn to understand, as it is the only way to show the working details of any piece of machinery. We have here a cylinder cover, a, which bolts firmly to the top of the cylinder, there being a similar one (generally without any stuffing-box) at the other end or bottom of the same. On the top of this you will observe another piece, which is marked b, and which is indeed part of the first and cast in one piece with it. Through the cylinder cover, a, is bored a hole of the exact size of the rod attached to the piston, which has to pass through it, but which hole, however well made, would allow the steam to leak considerably during the working of the piston-rod.
Fig. 57.
To obviate this, the part b is bored out larger, and has a cup-shaped cavity formed in it, as you will see by inspecting the drawings. Into this cavity fits the gland, c, which also has a hole in it, to allow of the passage of the piston-rod. This gland is made to fit into the cavity in b as accurately as possible; and it can be held by bolts as in the fig. A, or be screwed on the surface as shown at B, in which latter case the greater part of the interior of b is screwed with a similar thread. The piston-rod being in place, hemp is wound round it (or india-rubber packing-rings are fitted over it), and the gland is then fitted in upon it, and screwed down, thus squeezing the hemp or rubber tightly, and compelling it to embrace the piston-rod so closely, that leakage of steam is wholly prevented. Whenever you have, therefore, to prevent steam or water escaping round a similar moving-rod in modelling pumps or engines, you will have to effect it in this way. The piston was also packed with hemp or tow, either loosely-plaited or simply wound round the metal in a groove formed for the purpose. In Fig. 57, C and D, I have added drawings of a piston, so made, partly for the purpose of again explaining the nature of sectional drawings. In this one, C, you are shown the end of the piston-rod passing through the piston, and fastened by a screwed nut below, a shoulder preventing the rod from being drawn through by the action of this nut. The hemp packing is also shown in section, but in the drawing D the groove is left for the sake of clearness.