t this deposit,
even under the worst conditions, accumulates very slowly, and the
operation of cleaning out the water-jacket is a very infrequent
necessity. The exhaust valve, however, may become overheated if it is
allowed to get into bad condition, _i.e._, leaky. Its seat should be
well looked after, or the hot gases will blow past when it is
presumably shut; and if this defect, slight though it may be to begin
with, is allowed to develop, both the seat, the valve head, and the
spindle will become burnt away and pitted, perhaps badly, due to the
excessive heat.

CHAPTER IV

IGNITION DEVICES

The ignition devices commonly employed may be divided into three main
classes--the metal tube, the porcelain tube, and the electric ignition.
These again may be subdivided: The first being either iron or nickel
(hecknum as they are sometimes called); the second are of two
kinds--single-ended and double-ended; and the third takes many forms
which many of my readers are possibly well acquainted with, such as the
magneto, the induction coil and trembler, and the high-tension magneto
ignition, the latter device having been used successfully on various
occasions, though not yet universally adopted.

The first-named have one or two advantages over the nickel tube. They
are very inexpensive, and are easily heated to the required temperature;
moreover, they can be made at home, should occasion demand. On the other
hand, they are not so durable, have a very uncertain life, and
consequently need renewing frequently--their average life being not
more than 60 working hours. Fig. 13 gives an outline drawing of an iron
tube, with its burner and chimney fixed in position. The tube is very
similar to a piece of 1/4-in. gas-barrel, closed up at one end and a
taper thread (1/4-in. gas) cut on the other; in fact, gas-barrel may be
used for making these tubes at home--and measure about 7 or 8 in. over
all It is screwed into a firing block, which in turn is screwed into the
combustion chamber end, so that when right home it is in such position
that the tube stands quite vertical. The section of the tube, fig. 13,
shows the condition it gets into after having been in use some time. The
bore, it will be seen, has become almost completely closed up, so that
there is practically no communication between the hot part of the tube
and the combustion chamber. This closing up of the bore is very gradual,
and it is in the early stages of this proce