reme importance that
the provisions for ventilation are such that the crew of the
submarine should lose as little as possible in its efficiency and
comfort. A fair amount of speed both on and below the surface of the
water is essential and the maintenance of the speed for a fairly
long period of time must be assured.

In regard to their general outward appearance, submarines of various
types to-day vary comparatively little. In many respects they
resemble closely in shape, torpedo boats--the earlier submarines
particularly. In size, of course, they differ in accordance with the
purposes for which they have been designed. As compared with earlier
submarines the most notable difference is that modern submarines
possess more of a superstructure. Almost all of them are built now
with double hulls. The space between the outer and the inner hull is
utilized primarily for ballast tanks by means of which submergence
is accomplished and stability maintained and regulated. Some of
these tanks, however, are not used to carry water ballast, but serve
as reservoirs for the fuel needed by the engines. The stability of
the submarine and the facility with which it can submerge also
depend greatly on the distribution of weight of its various parts.
This problem has been worked out in such a way that to-day there is
little room for improvement. Its details, however, are of too
technical a nature to permit discussion in this place.

Hydroplanes both fore and aft are now generally used to assist in
regulating and controlling stability in the submerged state. The
motive power of the modern submarine is invariably of a two-fold
type. For travelling on the surface internal combustion engines are
used. The gasoline engine of former years has been displaced by
Diesel motors or adaptations of them. Although these represent a
wonderful advance over the engines used in the past there is still a
great deal of room for improvement. The opinions of engineers in
this respect vary greatly, American opinion being generally
unfavourable to the Diesel type, and whether the final solution of
this problem will lie in the direction of a more highly developed
motor of Diesel type, of an improved gasoline engine, or of some
other engine not yet developed, only the future can tell. Simplicity
of construction and reliability of operation are the two essential
features which must be possessed by every part of the power plant of
a submarine. For underwater trav