The simple plan of heating by hot water is that which Sir Joseph Paxton has adopted for the Crystal Palace. But simple as the method undoubtedly is, its adaptation to the purposes of the Palace has cost infinite labour and anxious consideration: for hitherto it has remained an unsolved problem how far, and in what quantity, water could be made to travel through pipes--flowing and returning by means of the propulsion of heat from the boilers. At Chatsworth, the seat of the Duke of Devonshire, the principle has been carried out on a large scale, and the experiment there tried has yielded data and proof: but in the present building, a greater extent of piping has been attached to the boilers than was ever before known, or even contemplated. In order to give the visitor some idea of the magnitude of the operation in question, it will be sufficient to state that the pipes for the conveyance of the hot water, laid under the floor of the main building, and around the wings, would, if placed in a straight line, and taken at an average circumference of 12 inches, stretch to a distance of more than 50 miles, and that the water in flowing from and returning to the boilers, travels one mile and three-quarters. But even with these extraordinary results obtained, the question as to the distance to which water can be propelled by means of heat is far from being definitely settled. Indeed, Sir Joseph Paxton and Mr. Henderson invented an ingenious contrivance, by means of which, should it ever be required, a much larger heating surface may be called forth at any time in any particular portion of the building. The general arrangement of the Heating Apparatus may be described as follows:--Nearly twenty-four feet below the surface of the flooring of the main building, and leading out of “Sir Joseph Paxton’s Tunnel” (the name given to the roadway in the basement story, extending the whole length of the building on the side nearest the Gardens), are placed, at certain intervals, boiler-houses, each containing two boilers capable of holding 11,000 gallons of water. The boilers are twenty-two in number, and are set in pairs. In addition to these, a boiler is placed at the north end of the building, on account of the increased heat there required for the tropical plants. There are also two boilers set in the lower stories of each wing, and two small boilers are appropriated to the water in the fountain basins at each end of the building, which contain Victoria Regias and other aquatic plants of tropical climes. Four pipes are immediately connected with each boiler; two of such pipes convey the water from the boiler, and the other two bring it back; they are called the main pipes, and are nine inches in diameter.