The bacteria necessary to carry on the process are always present in sewage, and after any particular plant has been in operation for a little while there results an accumulation of them, so that the process becomes more and more active as time goes on. Mechanical ingenuity has so arranged matters that a sewage disposal plant on this system can be made quite automatic, requiring little or no attention for months together, the raw sewage flowing in at one end, while the odourless, harmless effluent pours out at the other. And, moreover, so powerful is the action of these beneficent bacteria that should disease germs come down in the sewage they soon destroy them. No chemicals are needed, for the bacteria replenish themselves. No sludge is left, everything being turned into the harmless effluent. And, it may be said once more, disease germs are destroyed. Of all the valuable inventions of modern times this is surely not one of the least. CHAPTER XIX MODERN ARTILLERY Even as late as the time of the Crimean War guns, even the largest, were made of that extremely common material, cast-iron. In fact, so far as material went, there was no difference between a gun and a water-pipe. It was the need for some material possessing strength comparable with that of steel combined with the ease of production of cast-iron which led Sir Henry Bessemer to experiment in the manufacture of steel. Out of those experiments came Bessemer steel and its near relative, Siemens steel, two materials of universal application at the present time, so that to the needs of the artilleryman we owe two inventions which have proved of infinite value in peace as well as in war. If any particular piece of ordnance can be said to be the prime favourite with the English-speaking peoples, it is the big naval gun. With both British and Americans the navy takes pride of place; both nations are given to contemplating with pleasure the number of dreadnoughts which they possess, and the distinguishing feature of a dreadnought is the large number of big guns which it carries. Of the latest of these gigantic weapons one may not speak, but much is already public property concerning the 12-inch gun which the original _Dreadnought_ carried, and which is probably followed in its general features by the still greater guns of the most recent ships. A gun is spoken of by its "calibre," which means the inside diameter, or, to use another expression, the size of the "bore." So the "12-inch" naval gun is 12 inches in the bore. Its length is in some cases 45 calibres and in others 50 calibres. In other words, some are 45 feet long and others 50 feet.