title="" tag="{http://www.w3.org/1999/xhtml}img"/> Fig. 15.

The asbestos lining, shown in fig. 13, may be of various thicknesses, according to the size of the chimney and the tube; the reason for this will be apparent to many; but being a most important factor in the heating of the tube, and consequently the working of the engine, it will be advisable to deal with this point more fully.

Due mainly to the peculiar behaviour of iron tubes under heat and internal pressure, it is always advisable to look to them first of all when the engine shows signs of missing fire; and to always examine the bore of a fresh one, and ascertain that it is perfectly clear before putting it in. The adjustment of the ignition tube, although one of the most important and necessary to be made on the whole engine, is in itself a perfectly simple matter. It must be understood that the ignition tube cannot, with the ordinary means at our disposal, be kept at too high a temperature; but it must not be assumed that either the size of the flame, or the time the flame has been alight, is conclusive evidence that the tube is, or ought to be, sufficiently hot to fire the charge successfully. It is an uncommon thing to hear a man exclaim—after it has been pointed out that his tube is practically cold—"Why, it's been alight for hours!"

If such is the case with you, reader, you may very rightly assume that the burner is not properly adjusted, and so does not give the right kind of flame.

In order to get the hottest possible flame, the quantity of gas and air must be mixed in the right proportions. A common fault is that there is too much gas allowed to flow through the nipple, compared with the amount of air being drawn in at the air aperture, fig. 13. The result is, we get a flame of great length, but one which is not at all suited to our requirements; and instead of giving up its heat to the tube and the asbestos lining of the chimney, a large amount of gas we are presumably burning in the chimney is not being burnt there at all, for, on applying a light just above the chimney top, a quantity of this gas we are wasting will be seen to burn with a flickering blue flame.

To put matters right, it is necessary to do one of two things—either cut down the supply of gas or increase the air-supply. Providing the air aperture is normal, i.e., the same size as it was originally, it is better to adjust the gas, which may be done by tapping up the nipple N, as indicated in the enlarged sketch, fig. 14, until just the right amount of gas can flow.

As a rule, if there is too much air, the flame will burn with a loud roaring noise, and is liable to fire back. The nipple should then be opened out with a small reamer—the tang of a small file, ground to a long taper point, makes an admirable tool for this purpose. Whether the burner is of the ordinary bunsen type, or the ring or stove type, the above remarks apply, as in every case the flow of gas is governed by the size of the orifice through which it flows.

There is no need to use anything beyond a touch of oil when putting in a new tube, in order to make a perfectly tight joint; white or red lead are quite unnecessary, and are liable to make it a troublesome matter to remove the tube on future occasions. Neither should undue force be applied when putting in new tubes; it is liable to wear the thread in the firing block, which results in a partial stoppage of the ignition hole, as indicated in fig. 15. This is especially the case if we happen to get hold of a tube with its screwed part slightly smaller than usual.

The asbestos with which the chimney is lined should be about 1⁄8 in. thick, and, when renewing, the same thickness should be used as originally. A thicker board will reduce the annular space round the tube, and will have a choking effect on the flame—much the same as referred to above, when there is too much gas and not enough air. A simple method of lining the chimney is to cut a block of wood to the inside dimensions of the chimney, less 1⁄4 in. in width and thickness, then soften the asbestos cardboard by immersing in water, and bend it round the wood, cutting off to the required size, i.e., till the two edges form a neat butt joint. It can be allowed to remain on the mould until dry—when it will retain its shape—or can be put into the chimney straight away, if it is wanted for use immediately. In the latter case, however, it will be some fifteen minutes or so before the tube will attain its working temperature. Asbestos linings gradually become worn and ragged, and small flakes are apt to detach themselves and fall down into the burner, which, of course, prevents the flame playing as it should around the tube. In such cases it is not always necessary to fit a new lining; if the chimney is removed, the loose flakes shaken out and the asbestos well damped and patted down with a wooden or steel foot-rule or other suitably shaped tool, it will be fit for another long spell of work.

The nickel or hecknum tubes are treated in the same manner as the iron, but, as we mentioned before, are more durable, but require more heating to get them up to a workable temperature. Their greater first cost is compensated to some extent by makers in some cases guaranteeing them for six months.

Of the porcelain ignition devices, we will deal with the double-ended tube first, it being the more commonly used of the two in this country. This form of tube is usually about 3 in. long, 1⁄2 in. diameter, and open at both ends. It may be mounted in a metal casting, in form not unlike the small gas stoves for heating soldering irons. It is heated the greater part of its length by a couple of rows of gas jets, and is frequently surrounded by an asbestos lining. The whole arrangement is in reality a tiny furnace. When in position for working, one end of the tube is open to the ignition passage leading and communicating with the combustion chamber, while the other end is sealed, through butting up against a metal cap or plate. An asbestos washer is interposed between the tube at each end and the metal it bears against, thus making a more or less flexible joint. A thumb screw is arranged at the outside end of the tube, by means of which pressure can be applied to clamp it up between the washers to the desired extent. Some care has to be exercised in adjusting this form of tube for running. When heated to the working temperature it, of course, expands, so that, if tightened up too much when cold, it is under a fairly high compression; and when the engine is started, and the explosion takes place, it not infrequently bursts, if there is not sufficient "give" in the washers to allow for the expansion. On the other hand, if not clamped up sufficiently tight to start with, when the explosion occurs, the washer at one or each end is blown out. This adjustment has to be made to a nicety, and, although a somewhat difficult matter, success may be attained after one or two trials. It is advisable, after a new tube has been put in, to start up the engine gently, i.e., with less than the normal supply of gas, and increase to the full amount gradually whilst running. This may be done by simply opening the gas-cock on engine partially in the first place.

The single-ended porcelain tube is not so well known here as on the continent; why, we cannot say; certainly it is preferable in every way. We give a few illustrations, showing the method of using this tube.

Figs. 16 and 17 show the general