ntrol marine propulsion. Instances are on record of
vessels having engines without marine governors being detained by
stress of weather at the mouth of the Thames, while vessels having
such governors, of good design, have gone to Newcastle, have come
back, and have found the other vessels still waiting for more
favorable weather.

With respect to condensation in marine engines, it is almost
invariably effected by surface condensers, and thus it is that the
boilers, instead of being fed with salt water as they used to be,
involving continuous blowing off, and frequently the salting up, of
the boiler, are now fed with distilled water. It should be noticed,
however, that in some instances, owing to the absence of a thin
protecting scale upon the tubes and plates, very considerable
corrosion has taken place when distilled water, derived from
condensers having untinned brass tubes, has been used, and where the
water has carried into the boiler fatty acids, arising from the
decomposition of the grease used in the engine; but means are now
employed by which these effects are counteracted.

LIGHT ENGINES AND BOILERS.

I wish, before quitting this section of my subject, to call your
attention to two very interesting but very different kinds of marine
engines. One is the high-speed torpedo vessel, or steam launch, of
which Messrs. 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