f damaged or sunk submarines. At the same time the navies
of the world have also produced special submarine tenders or mother
ships. The purpose of these is to supply a base which can keep on
the move with the same degree of facility which the submarine itself
possesses. These tenders are equipped with air compressors by means
of which the air tanks of submarines can be refilled. Electric
generators make it possible to replenish the submarine storage
batteries. Mechanical equipment permits the execution of repairs to
the submarine's machinery and equipment. Extra fuel, substitute
parts for the machinery, spare torpedoes are carried by these
tenders. The most modern of them are even supplied with dry dock
facilities, powerful cranes, and sufficiently strong armament to
repel attacks from boats of the type most frequently encountered by
submarines.

There are, of course, many other special appliances which make up
the sum total of a modern submarine's equipment. Electricity is used
for illuminating all parts of the boat. Heat is supplied in the same
manner; this is a very essential feature because the temperature of
a submarine, after a certain period of submergence, becomes
uncomfortably low. Electricity is also used for cooking purposes.

Every submarine boat built to-day is equipped with wireless
apparatus. Naturally it is only of limited range varying from one
hundred and twenty to one hundred and eighty miles, but even at that
it is possible for a submarine to send messages to its base or some
other given point from a considerable distance by relay. If the
submarine is running on the surface of the water the usual means of
naval communication-flag signals, wig-wagging or the semaphore, can
be employed. The submarine bell is another means for signalling. It
is really a wireless telephone, operating through the water instead
of the air. Up to the present, however, it has not been sufficiently
developed to permit its use for any great distance. It is so
constructed that it can also be used as a sound detector.

Some submarines, besides being equipped with torpedo tubes, carry
other tubes for laying mines. In most instances this is only a
secondary function of the submarine. There are, however, special
mine-laying submarines. Others, especially of the Lake type, have
diving compartments which permit the employment of divers for the
purpose of planting or taking up mines.

Disappearing anchors, operated by electricit