e user years ago no longer exist.

All that we require of the cooling water is that it shall keep certain
working parts of the engine at a reasonable temperature; for instance,
the cylinder must not be so hot as to deprive the lubricating oil of its
property to lubricate, neither must the exhaust valve become so hot as
to cause it to seize in the bush and stick up; but, beyond such
considerations as these, the higher the temperature is at the
commencement of each explosion the more efficient will the engine be.
The object, then, is to do as little cooling as possible, and to apply
the cooling effect at the right parts; hence the passages and chambers
through which the cooling water circulates should be so arranged that
those which require to be kept at a low temperature are in close
proximity to the cooling water. On some of the engines of days gone by,
the exhaust valve was carried in a large iron casting, this in turn
being bolted to the cylinder casting and communicating with the
combustion chamber by means of a port. Such an arrangement was found to
be not only clumsy but inefficient; the water passages were small and
difficult to get at; they readily furred up; and moreover, the joint
between this casting and the cylinder was necessarily a water _and_
explosion joint, and the fewer we have of these the better.

The method--if it may be called a method--of overcoming or preventing
the exhaust valve becoming too hot is, in the case of figs. 11 and 12,
simply one of judicious arrangement and design. The cooling water enters
by the inlet K (fig. 11), and circulates round the exhaust valve port X
and valve E immediately, before becoming heated, thus keeping the
hottest of the working parts of the engine at a suitable temperature;
and the valve seat, being in direct metallic communication with the cold
water, does not become burnt or pitted. On the other side of the exhaust
valve we have the air valve and its passages, through which cool air is
continually being drawn; this also helps to keep the exhaust valve cool.

From this, then, we may conclude that overheating of the cylinder will
not occur under normal conditions, given an engine of good design; but,
if this trouble does arise, we may safely look first of all for some
defect in the cooling water circulation. Some waters contain a greater
amount of impurities than others, and consequently the water space may
furr up more rapidly in one district than in another. Bu