Briefly, it enables a steady and continuous stream of water to be thrown on a fire. It is the vital principle of the modern fire-engine, and renders it distinctly different from all squirts, syringes, and portable pumps preceding it. Instead of an unequal and intermittent supply, sometimes, no doubt, falling far short of the fire, we have now a persistent stream, which can be continuously directed to any point, in reach, with precision and efficiency. How, then, are these results obtained? How does the air-chamber work? It depends on the elasticity and power of compressed air. The water, when drawn from the source of supply by two pistons, working alternately, is driven into a strong chamber filled with air. The air becomes compressed, and is driven to one part of the chamber; but when it is forced back to occupy about one-third of the whole space, the air is so compressed that, like the proverbial worm which will turn at last, it exerts a pressure on the water which had been driving it back. If the water had no means of escape, the chamber would soon burst; but the water finds its way through the delivery-hose. If the hose issue from the top of the chamber, it is fitted with a connecting pipe reaching nearly to the bottom to prevent any escape of air. Now, as long as the pumps force the water into the air-chamber to the necessary level--that is, to about two-thirds of the space--the pressure is practically continuous, and thus a constant jet of water is maintained through the hose. The ordinary pressure of air is about 14·7 pounds per square inch; and when compressed to one-half its usual bulk, its elasticity or power of pressure is doubled, and of course is rendered greater if still further compressed. This power, then, of the compressibility and elasticity of air is the secret of the fire-engine air-chamber; but though introduced about 1675, it was not until 1720 that such engines seem to have become more general. About that date, Leupold built engines in Germany with a strongly-soldered copper chest, and one piston and cylinder, the machine throwing a continuous and steady jet of water some twenty or thirty feet high. In the meantime, what was being done in England? Here again the story is obscure; but we imagine the course of events to have been something like this: In the dismal days after the Great Fire, people began to cast about for means to prevent a recurrence of so widespread and terrible a calamity. Fire-insurance offices were organized, and they undertook the extinguishment of fires. It is not unreasonable to suppose that in some form--perhaps by offering prizes, perhaps by simply calling attention to the need for improvement, perhaps by disseminating information such as of the engine mentioned by Perrault at Paris--these offices stimulated invention; perhaps the memory of the Great Fire was enough to stir ingenious effort without their aid.