things occurred, but that the submarine's delicate electrical machinery
was thrown out of order by the violence of a depth-charge explosion,
even when a considerable distance away. With the electric engines
used for submerged propulsion no longer available, and possibly the
interior of the vessel in darkness, there were only two courses open.
She could either rise to the surface and endeavour to fight it out with
the aid of her powerful deck guns, or else sink to the bottom and trust
to luck that other depth charges would not be dropped close enough to
seriously damage her hull. In the open sea, however, the latter chance
was denied because of the depth of water. Three hundred feet may be
taken as the greatest depth to which an ordinarily constructed fighting
submarine can safely descend without running a grave risk of having her
plates crushed in by the great water pressure. Even at this depth the
weight on every square foot of hull surface exceeds 8 3/4 tons.

If the damaged submarine rose to the surface the guns of her pursuers
were ready and could generally be relied upon to place her at least
_hors de combat_ before the hatches of the under-water vessel could be
opened and her own guns brought into action.

[Illustration: FIG. 9.--Diagram showing how depth charges are carried on
the stern of a motor launch. _AA._ Depth charges, each containing 300
lb. of high explosive. _B._ Hydrostatic device by means of which the
charge can be made to explode when it has sunk from the surface to a
depth of 40 or 80 feet, and by which it is rendered comparatively safe
while on deck. _C._ Slings holding charges in place on inclined
platform. _D._ Greased bolts which, on being pulled out, allow wire
slings to fly free and depth charge to roll into the sea. Depth charges
can only be released from vessels under way, otherwise the explosion
which occurs a few seconds after release damages surface vessel.]

In shallow water where there was a fairly smooth bottom it generally
happened that a submarine damaged by depth charges elected to sink to
the sea-bed and trust to luck. This was also frequently resorted to as a
means of eluding pursuit even when the U-boat was not damaged by the
first few charges dropped. It was then that the hydrophones carried by
the surface ships were again brought into use to ascertain if the
submarine was still under way. When no sound was heard those on the
surface knew that "Fritz" was lying doggo, or else