Experience had made that dull roar familiar to all on board, and it needed no order from the now hard-faced C.O. to cause every man to don his "capuc" life-belt in readiness for the hidden dangers which they knew to be strewn in the pathways of the sea ahead. Mines are moored at a given depth below the surface, usually from six to ten feet. The rise and fall of the tide, therefore, either increases or decreases the stratum of free water above them. This causes these invisible submarine weapons to be more dangerous to shallow-draught vessels, such as motor patrol launches, at low tide, when there is little water between the tops of their horns and the surface, than at high tide. More will, however, be said in a later chapter about mines and the difficulties of laying them. It so happened that on this occasion the tide was low and the mines consequently extremely dangerous to even the shallowest draught type of warship. The speed of the M.L. was increased until the twin engines were revolving at the rate of 490 a minute. The snow haze seemed suddenly to grow thicker and all around the flurries of white blotted out the distant view. The minutes of pounding through the slate-grey seas seemed interminably long, and the flying clouds of icy spray stung every exposed part of the human frame. When about three sea miles had been traversed the engines were stopped and all on board listened for a cry from the sea ahead. The C.O. pulled the peak of his drenched cap farther over his eyes and gazed out into the opaque greyness ahead. Minutes passed; but little ships cannot rest quietly on the open sea. The lash of the water and the slapping of the meagre rigging drowned any faint sound there might have been, and once more the engines throbbed to the order "Slow ahead!" Barely had the ship gathered way before a dark object appeared momentarily in the trough of the sea about two degrees on the starboard bow and the next instant seemed swallowed up. A warning cry from the look-out on the tiny sea-washed fo'c'sle head, a sharp order from the bridge, and, within its own length, the patrol boat swung rapidly to port. At the same moment a dan-buoy splashed overboard to mark the position of the floating mine. A few yards more to the eastward and No. 822 would have appeared in the list of the missing. Minutes of tense nerve strain followed, for all knew that the ship was in the midst of a mine-field, and the deadly horns which had been momentarily visible on the surface were but a single example of the many which lurked around. Eyes were strained into the grey-green depths, and yet all knew the impossibility of seeing. Again the look-out's warning cry and the engines were reversed, but this time it was not a mine, but the victim of one, holding on to a piece of wreckage.