Thornycroft's firm have furnished so many examples. In
these, owing to the rate at which the piston runs to the initial
pressure of 120 lb. and to very great skill in the design, Messrs.
Thornycroft have succeeded in obtaining a gross indicated horse-power
for as small a weight as half a cwt., including the boiler, the water
in the boiler, the engine, the propeller shaft, and the propeller
itself.
To obtain the needed steam from the small and light boiler, recourse
has to be made to the aid of a fan blast driven into the stoke-hole.
From the use of a blast in this way advantages accrue. One is, as
already stated, that from a small boiler a large amount of steam is
produced. Another is that the stoke-hole is kept cool; and the third
is that artificial blasts thus applied are unaccompanied by the
dangers which arise, when under ordinary circumstances the blast is
supplied only to the ash-pit itself.
THE PERKINS SYSTEM.
The second marine engine to which I wish to call your attention is one
that has been made with a view to great economy. The principles
followed in its construction are among those suggested by the
President (Sir W.G. Armstrong) in his address. He (you will remember)
pointed out that the direction in which economy in the steam engine
was to be looked for was that of increasing the initial pressure;
although at the same time he said that there were drawbacks in the
shape of greater loss, by radiation, and by the higher temperature at
which the products of combustion will escape.
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