ing pressures, where these
are materially different, than directly to the indicated power. The
advantages of saving weight of machinery, so long as it can be done with
efficiency, are well known and acknowledged. If weight is to be reduced,
it must be done by care in design, not by reduction of strength, because
safety and saving of repairs are much more important than the mere
capability of carrying a few tons more of paying load. It must also be
done with economy; but this is a matter which generally settles itself
aright, as no shipowner will pay more for a saving in weight than will
bring in a remunerative interest on his outlay. In his paper on the
weight of machinery in the mercantile marine,[3] Mr. William Boyd
discussed this question at some length, and proposed to attain the end
of reducing the weight of machinery by the legitimate method of
augmenting the speed of revolution and so developing the required power
with smaller engines. This method, while promising, is limited by the
efficiency of the screw, but may be adopted with advantage so long as
the increase in speed of revolution involves no such change in the screw
as to reduce its efficiency as a propeller. But when the point is
reached beyond which a further change involves loss of propelling
efficiency, it is time to stop; and the writer ventures to say that in
many cargo vessels now at work the limit has been reached, while in many
others it has certainly been passed.

[Footnote 3: Transactions Northeast Coast Institution of Engineers and
Shipbuilders, vol. 6, 1889-90, p. 253.]

_Economy of Fuel_.--Coming to the highly important question of economy
of fuel, the average consumption of coal per indicated horse-power is
1.522 lb. per hour. The average working pressure is 158.5 lb. per square
inch. Comparing this working pressure with 77.4 lb. in 1881, a superior
economy of 19 per cent. might be expected now, on account of the higher
pressure, or taking the 1.828 lb. of coal per hour per indicated
horse-power in 1881, the present performance under similar conditions
should be 1.48 lb. per hour per indicated horse-power. It appears that
the working pressures have been increased twice in the last ten years,
and nearly three times in the last nineteen. The coal consumptions have
been reduced 16.7 per cent. in the last ten years and 27.9 per cent. in
the last nineteen. The revolutions per minute have increased in the
ratios of 100, 105, 114; and the piston s