Despite all precautions, many merchant vessels were mined in the War-Channel in the course of the war; but these disasters were largely due to the carelessness of shipmasters, who at times neglected to comply with the instructions that had been given to them. How well the Harwich auxiliary vessels carried out their work, and how heavy that work was, the following figures show. In the year 1917, the total number of enemy mines swept up and destroyed by the mine-sweepers of the thirty-three bases of the British Isles amounted to 3400, of which over 1000 stand to the credit of the Harwich base. It is a notable fact, too, that in the same year 500 mines were destroyed consecutively in this area without the loss of a single merchantman, whereas the average for the United Kingdom had been one merchantman lost to thirteen mines destroyed. CHAPTER XII WORK OF THE AUXILIARIES CHAPTER XII WORK OF THE AUXILIARIES Mine-sweeping methods--Indicator nets--Heavy losses--Brilliant rescues. Without going into technical details, I will now give a brief explanation of the usual methods employed by the mine-sweeping trawlers of the Harwich base. Two trawlers steaming abreast at about four hundred yards distance apart tow a sweep wire eight hundred yards in length, an end of which is attached to each trawler. The wire thus drags astern in a great loop, which is kept at the requisite depth--that is, at a depth well exceeding the draught of the deepest ship which would travel across that area--by kites. This sweep wire is serrated, so that when towing it quickly saws through the moorings of the mines, which are thus released and rise to the surface. When two or more pairs of trawlers are sweeping in unison they adopt what may be termed an echelon formation. The second pair of mine-sweepers follows the first pair, at a safe distance astern, on a parallel course, but on an alignment that causes the space swept by the following pair of vessels to somewhat overlap that swept by the leading pair, so that no unswept space is left between the two. If a third pair of vessels follows, it takes up a similar position astern of the second pair; and so on, if there be other pairs engaged in the sweep. When a strong cross tide is running, to carry out this operation accurately is no easy task. But the skilled North Sea fishermen who man the trawlers are the right men for this sort of work. They rapidly acquire all the tricks of sweeping, and soon learn to detect a mine that has been caught in the sweep by the singing of the sweep wire, the feel of it, and other delicate signs. The mine-sweeping trawlers are accompanied by a vessel whose duty it is to sink or explode by rifle fire the released mines as they appear on the surface.