f
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 arrangement of tube and chimney and the
manner in which they are fixed to the cylinder. The device consists
primarily of three parts--the body or chimney B, the cover C, and the
tube itself T. The body is a light iron casting, carried by a couple of
studs SS, which are either screwed into the firing block F, or direct
into the metal of the cylinder casting if no firing-block is used; the
latter may very well be dispensed with in the smaller-sized engines.

The tube is made of thin porcelain, slightly bell-mouthed at its open
end, and is mounted in a thick metal washer W, as shown in fig. 18 in
section, the joint being made with a little asbestos paper, moistened.

The block F and the face of the body B (fig. 16) are recessed to take
the washer W easily, but the depth of both recesses taken together must
be about 1/16 in. less than the thickness of the washer W; thus, when
the tube is placed in position between the body B and the block F, and
the former screwed up by means of the two nuts, as shown in the figure
16, the effect is to clamp the _washer_ which carries the tube, but _not
the porcelain tube itself_.

[Illustration: FIG. 16.]

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