ss that erratic firing is
likely to occur; sometimes the charge will be successfully fired and
sometimes not. It may be as well to mention here that the length of the
tube, although to a certain extent immaterial, should neither be
excessively long nor abnormally short, the precise length varying with
the size of the engine. A 1/4-in. tube, 8 ins. long, may be used
successfully on engines ranging from 1/2 to 6 horse-power, provided a
suitable burner is fitted enabling the tube to be heated at any required
spot. After the first charge has been fired, and the exhaust takes
place, practically all the burnt gases are cleared out of the cylinder,
but a small amount of these will generally remain in the tube and the
bore of the firing block. On the ensuing compression stroke these inert
gases are compressed to the far end of the tube, thus making way for the
explosive mixture to reach the hot portion, and explode, thus sending a
jet of flame into the main volume of the mixture which is immediately
ignited. Hence there is no advantage in having a tube too long, while,
on the other hand, it _must_ not be too short.

[Illustration: FIG. 13.]

[Illustration: FIG. 14.]

[Illustration: 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 ali