Rapidly moving detachments of cavalry are provided with what is called "air-raft equipment," which enables them to get their light "Horse Artillery" guns across rivers which would be impassable otherwise. It consists of sixty bags like huge cylindrical footballs except that the outer covering is canvas instead of leather. These are blown up partly by the mouth and partly by pumps provided for the purpose until they are just about as tight as a football should be. Then they are laid out in rows of twelve, each row being fastened together by the bags being tied to a pole running lengthwise of the row. Cords are attached to the bags for the purpose. The five rows are then placed parallel and connected together by two light planks called wheelways placed across the rows and tied thereto. This arrangement is capable of carrying light guns or ammunition wagons. The men are expected to ride through the water, but if necessary something can be laid upon the raft, between the wheelways, to form a floor upon which men and even horses can ride. As part of the equipment there is a small collapsible boat with oars and by its means men first cross, carrying with them a line by which, afterwards, the raft can be hauled to and fro. Rafts can be made, too, of hay tightly tied up in waterproof ground-sheets or tarpaulins or canvas. Indeed, given a little ingenuity and the need to use it (for it is very true that necessity is the mother of invention), it is surprising what a large variety of things can be pressed into this service. Of course, barrels can be made to form excellent pontoons, but there is one clever little way of using them which is more than usually interesting, and with that I must conclude this chapter which has already exceeded its appointed limits. Imagine two poles perhaps ten feet long, placed parallel. Between them, at one end, a barrel is lashed: at the other end is a plank forming with the poles a T. A man can then sit upon the barrel and paddle about, for the poles and planks will steady the barrel just as the outriggers and floats steady the narrow canoes or catamarans of which we read in books of travel. For that reason a bridge formed of such is called a "catamaran" bridge. Of course, if there are only a few barrels to be had they can be fitted out like this and then combined into a raft. Or if there are enough of them they can be anchored at intervals and poles or planks laid from one to another so as to form a continuous bridge. Or a single one may be used as a boat. I can almost fancy I see some of my readers who have access to a pond rigging up an old barrel in this way, just to see how it goes. CHAPTER VII WHAT GUNS ARE MADE OF No longer ago than the days of the Crimea, the largest guns were made of the cheapest and commonest kind of iron, that known as cast iron.