Page:Popular Science Monthly Volume 31.djvu/666

648 gun out. If it be desired to let the gun run out automatically immeniately after recoil, it is only necessary to leave the running-out cock F open, and then the water forced among the cork by recoil returns instantly to the cylinders, and runs the gun out quicker than the eye can follow the motion.

The arrangement adopted may be made by using air instead of cork, but air is a troublesome substance to deal with; it leaks out very easily and without showing any signs of having done so, which might readily lead to serious consequences. A special pump is required to make up loss by leakage.

The merit of cork is its extreme simplicity and trustworthiness. By mixing a certain proportion of glycerine with the water it will not freeze in any ordinary cold weather.

Each of the applications of cork is based on some of the physical or chemical properties of the substance. In bottle-corks its impermeability, elasticity, and imputrescibility are brought into service. Its lightness, the first quality that strikes us, on a superficial view, is not considered.

Cork is used for a variety of other purposes than those which have been mentioned, which, while not so economically important as these, still deserve attention. The male cork, while not well adapted for stoppers, has been made available in the decoration of parks and gardens. Rice-hulling mills have been made from it, but not with much success; small corks can be got out of it. Water-conduits and bee-hives have been constructed from it. It furnishes excellent damp-proof shelves and stands. The Kabyles employ it, mixed with a mortar of mud, in building the walls of their houses, and shingle their roofs with it. It is used for the floats of fish-nets.

These various applications were known, as we learn from expressions of Theophrastus and Pliny, to the Greeks and Romans. Pliny says: "Only that bark is used that is thick and springs back when it is pulled. It is sometimes employed for the buoys of ships' anchors, fishermen's nets, barrel-bungs, and women's winter sandals. The Greeks felicitously called the cork-oak the bark-tree. Cork is used for the covering of roofs." The chips form a good non-conducting material for keeping ice, and reduced to fragments make excellent stable-floors and race-course tracks.

The real, or female cork, has a more homogeneous grain and works much better than male cork. It is a very poor conductor of heat and sound, and has been found valuable to protect hot surfaces against cooling, and to keep frigid substances from melting. It is the basis of several non-conducting mastics and coverings, which are used for protecting pipes, steam-boilers, hot-water reservoirs, etc. Three methods of applying the cork-covering are employed in France. Strips of cork touching at their edges may be laid along the pipes and cylinders and drawn together by wire as in No. 1, Fig. 9. The pipe clothed in