ight of the water, and the officer employs delicate
apparatus for these measurements.

On the open seas these alterations in weight do not occur very
rapidly; but whenever a boat approaches the mouth of a river, then the
transition from salt to fresh water happens very suddenly and may
provoke the undesirable disturbances to which we have already alluded.
Also warm and cold currents at different depths produce thermotic
conditions, which surprisingly change the weight of the water.

Peculiar as it may appear, a submarine must be lightened to descend to
a very great depth, whereas, in steering to a higher level, more water
must be admitted into the tanks to prevent our emerging to the surface
with too great suddenness. This demands careful attention, skill, and
experience.

The principal condition for the success of a submarine attack is to
steer to the exact depth required. The periscope must not rise too
far above water, for it might easily be observed by the enemy; but if,
by clumsy steering, the top of the periscope descends below the waves,
then it becomes impossible to take aim to fire the torpedo. The
commander therefore must be able to depend on the two men who control
the vertical and horizontal rudders, whom another officer constantly
directs and supervises.

When the boat has reached the prescribed depth a close examination is
made of all the outward-leading pipes, to see if they can properly
resist the water pressure; if any tiny leak has been sprung, every cap
must be tightly screwed down; for it is evident it would be very
undesirable if any leak should occur and increase the heaviness of
the submarine. Absolute silence must prevail so that any dripping or
greater influx in the tanks can be observed.

Quietly and silently the boat advances against the enemy; the only
audible sounds are the purring of the electric motors and the
unavoidable noise made by the manipulation of the vertical and
horizontal rudders. Alert and speechless, every man on board awaits a
sign from the commander, who is watching in the turret; but some time
may elapse--now that the periscope is lowered and nearly on the level
of the waters--before the adversary becomes visible again. The ship
may have changed her course and have taken an opposite direction to
the one she was following at the moment we submerged. In that case she
would be out of reach and all our preparations prove useless.

At various intervals, the commander pres