On the top of the body was built a superstructure, which looked like a high box--greater in height than in breadth, and larger at the top than at the bottom. This box contained the all-important air-chamber and the pumps. The water in the interior pipe was forced into the air-chamber by the two pumps, and then thrown on the fire through a pipe connected with a hose of leather projecting from the top of the air-chamber. This pipe descended within the chamber almost to the bottom, so that when water was pumped into the air-chamber it flowed round the bottom of the pipe, and prevented any ingress or egress of air. As the water rose, the air already in the chamber became compressed in the top part of the chamber, and in turn exerted its power on the water. The pumps were worked by levers, one on each side of the engine, and alternately raised and lowered by the men operating the machine; while this manual-power was much increased by one or two men working treadles connected with the levers, and throwing the weight of the body on each treadle alternately. The principle of the force-pump may be thus briefly explained: When a tight-fitting piston working in a cylinder is drawn upward, the air in the cylinder is drawn up also, and a partial vacuum created; if the cylinder is connected with water not too far distant by a pipe, the water will then rush upward to fill the vacuum. Then, if the bottom of the cylinder be fitted with a valve opening upward only, it is closed when the piston is pushed down again; and the water would burst the cylinder, if enough power were applied to the piston, but escape is afforded along another pipe as an outlet, which in the case of the fire-engine opens into the air-chamber, and which is opened and closed by another valve. Thus is the water not only raised from the source of supply, but is forced along another channel. And the modern fire-engine--which we date from Newsham's engines in England about 1726--is a combination of the principles of the force-pump and of the air-chamber, which acts by reason of the great elasticity of compressed air. Other inventors made improvements as well as Newsham, namely, Dickenson, Bramah, Furst, Rowntree, and others, though the differences were chiefly in details. An engraving mentioned in an old work of reference sets forth that a London merchant named John Lofting was the patentee and inventor of the fire-engine. His invention must have been since the Great Fire, because the Monument is depicted in one corner of the engraving and the Royal Exchange in another. Rowntree made an engine for the Sun and some other fire-offices, which protected the feed-pipe more efficiently from mud and gravel; and Bramah devised a hemispherical perforated nozzle, which distributed water in all directions, so that the ceilings, sides, and floor of a room would become equally drenched.