Fish guano or fish manure is generally prepared from the fish waste discarded by the curer. An average sample of this manufactured fish manure will contain 12 per cent water, 60 per cent organic matter, yielding 10 per cent ammonia, 16 per cent of calcium phosphate, and a residue of salt, sand, magnesia and potash, the amount of potash being inconsiderable. Fish guano is mainly valuable as a source of ammonia, the ammonia content ranging from 6 to 11 per cent, according to the kind of fish used and its previous history, e.g. whether fresh or salted. In many places, such as London, the fish offal from the shops and restaurants is collected, dried and ground up for use as manure. In Germany in 1918 herrings’ heads were removed by the curers to be utilized for the production of oil, albumen, and phosphate of lime. The herring meal contained up to 50 per cent of albumen and calcium phosphate, the latter being obtained from the bones and heads. The albumen was extracted chemically and prepared for human consumption. The oil was extracted with benzol or other solvents, and, after hardening, was used in the manufacture of butter substitutes. Fish waste or offal is fed into a continuous cooker. This cooker consists essentially of a long, cylindrical vessel, through which runs a hollow steel shaft on which are mounted perforated radial vanes in such a way that the whole arrangement forms a spiral conveyor. By means of the hollow shaft and vanes, steam is blown into the mass of fish waste as it travels slowly through the vessel, so that it is completely cooked and disintegrated by the time that it emerges at the other end. The cooked mass is then fed into a press in which a screw conveyor urges it through a gradually tapering cylinder with perforated sides. In this way the oil is extracted from it, and it is then dried and disintegrated by a rotary drier. There is always a little residual oil in fish manure that tends to delay its decomposition in the soil. It is important, therefore, that the oil be removed as completely as possible. Dry fish manure requires careful storing, as the presence of this small amount of oxidizable oil tends to promote spontaneous combustion. In addition to its value as a fertilizer, the high content of protein (albumen)—namely, 50 per cent—makes fish meal a suitable food for live-stock and poultry. The commercial importance of this industry will be realized when we remember that practically half of the total catch of fish in the world is discarded by the curers as waste. =Fish Leather.= The hides of such marine mammals as the walrus and the seal have long formed the basis of a regular tanning industry.