in the whole cylinder casting there should be no
sudden change, but a uniformity in the thickness of the metal employed.
This point should be carefully remembered, although it applies more
particularly to those parts of the casting subjected to higher
temperatures than the rest.

The main bearings are usually of brass or gun-metal, and are adjusted
for running in the same manner as any steam or other engines would be.
The "brasses" are in halves, and are held down by the cast-iron caps, as
shown in fig. 1.

These bearings require extremely little attention, and do not show the
wear and tear of running nearly so soon as the connecting-rod brasses.
These, too, are usually of brass or gun-metal; but there are various
forms of construction employed in connection with the back end or piston
pin bearings. On very small engines the connecting rod is swollen at the
back end in the forging, and then machined up and drilled, as shown in
fig. 3. In this hole the brasses are inserted after being scraped up to
a good fit on the piston pin.

A flat is cut on one of the brasses, and a set screw is fitted, as
shown, to prevent any movement of the latter after the final adjustment
has been made. A lock nut should be used in conjunction with this set
screw. Another method, and one more generally used on larger engines, is
shown in fig. 4. In this case the brasses are larger than in the former,
where they are virtually a split bush; here they have holes drilled in
them to take the bolts, the latter usually and preferably being turned
up to the shape shown in fig. 5.

CHAPTER III

HOW A GAS ENGINE WORKS

The gas engine of the present day, although from a structural point of
view is very different to the early engine, or even that of fifteen
years ago, is, in respect to the principle upon which it works, very
similar. The greater number of smaller power engines in use in this
country work on what is known as the Otto or four-cycle principle; and
it is with this class of engine we propose to deal.

Reference to the various diagrams in the text will help considerably,
and make it an easy matter for any reader hitherto totally unacquainted
with such engines to see why and how they work.

Coal-gas consists primarily of five other gases, mixed together in
certain proportions, these proportions varying slightly in different
parts of the country:--Hydrogen (H), 50; marsh gas (CH4), 38;
carbon-monoxide, 4; olefines (C6H4), 4; nit