In addition to all these types of anti-submarine craft there were, forming part of the auxiliary fleet, over 300 ships, mostly trawlers and drifters, engaged in maintaining the great lines of boom defences, closing vast stretches of sheltered waters frequented by the battle fleets, and a considerable number of examination ships, staffed by interpreter officers, whose duty it was to examine all neutral shipping passing through the 10,000 miles of the blockade. * * * * * These, then, were the ships of the new navy, and their formation into flotillas, or units, was usually accomplished by grouping four or five vessels of similar type together under the command of the senior officer afloat--mostly a lieutenant R.N.R. or R.N.V.R. In the case of minesweepers the unit nearly always consisted of an even number of ships, because their work was carried out in pairs, and with M.L.'s it usually consisted of five boats, as this was the number required for the intricate tactical work of submarine chasing. There were, of course, units from the United States, French, Japanese, Italian and Brazilian navies, in addition to the formidable British armada. The auxiliary units were all based on one or other of the fifty odd war stations which encompassed not only the coasts of Great Britain and Ireland, but also the littoral of every land in our world-wide Empire. The numbers given here do not include the local fleets of purely colonial naval bases, nor the large flotillas of destroyers and "P" boats operating in home and foreign waters in conjunction with the auxiliary navy. If these were incorporated the anti-submarine fleets would be almost doubled. Now that the reader is familiar with the _raison d'etre_ of the new navy, the personnel, the ships and their formation into fleets, the scope and limitations of their activity, and of the losses they sustained, the way is clear for a description of the curious weapons used, the mysteries of anti-submarine warfare, and the bases themselves before entering the zone of war and seeing something of the actual work of the auxiliary navy. FOOTNOTE: [3] _Yachting Monthly_ and _R.N.V.R. Magazine_, August, 1917. CHAPTER V THE HYDROPHONE AND THE DEPTH CHARGE OF all the weapons used in the anti-submarine war the two most important were the _hydrophone_ and the _depth charge_. They were employed in conjunction with each other and comprised the surface warship's principal means of offence against submarines operating beneath the surface. The hydrophone resembles a delicate telephone. It is so constructed that when the instrument is lowered over the side of a ship into the sea any noise, such as the movement of a submarine's propellers, can be heard on deck by an operator listening at an ordinary telephone receiver connected to the submerged microphone by an electrified wire.