The mushroom sinker or anchor, which is undoubtedly the most effective of all other forms of mooring anchors used for the purposes of anchoring submarine mines, is shown at _e_, Fig. 49; the legs are added for use on rocky or hard bottoms, under which circumstances the weight of the anchor should also be increased. For ground mines the form of sinker shown at _d_, Fig. 48 is employed; it is of an oblong shape, and hollowed out in the centre to allow of its being lashed close up to the mine. Large blocks of stones with their bases slightly hollowed are useful as extempore moorings, so also is the one shown at Fig. 51, which consists of a strong heavy wooden shaft _a_, with a number of wooden arms _b_, _b_ attached to its base; this form of extempore sinker was considered very efficient by the American authorities. The wooden weighted platform, which was described at page 56, is also a very useful form of extempore sinker. For dead weight moorings, pigs of ballast, heavy stones, &c., may be used. The weight of the anchor or sinker for mooring submarine mines is a very important consideration. It will depend on the amount of buoyancy of the mine, on the strength of current, and on the nature of the bottom, also whether the mines are to be hauled down to, or moored with the anchor. Stotherd uses the following formula: W = [2rt](B^{2} + P^{2}) where B is the excess of the flotation over the weight of the charge of a given submarine mine; P is the pressure exerted by any given current on the same buoyant mine; W the weight of sinker necessary to overcome the tendency of the mine to move. In still water P becomes nothing, and therefore W equal to 2 B, that is, in still water double the buoyancy of a mine is a sufficient weight for its anchor. The value of P may be found from the formula P = 4·085 × V^{2}, where V is the velocity of the current in miles per hour. From this equation P will be found in terms of pressure in pounds per square foot of flat surface, which is nearly double that on the curved surface of a cylinder. In regard to the amount of buoyancy of a submarine mine, it has been found by actual practice that in the case of a mine moored in still water it should certainly be not less than the weight of the charge, whilst if subjected to the lateral pressure due to a current, it should be not less than three times the pressure exerted by the current. It is always necessary to allow an excess of buoyancy over the calculated amount to counteract any leakage, or other disturbing cause which might otherwise materially affect the efficiency of the mine. There are two modes of placing a mine in position; either by attaching the anchor, with the cable necessary for the depth of water, to the mine, and lowering both together, or by placing the anchor first, and then hauling the mine down to it, and by means of a catch, fastening it at the required depth.