In this apparatus the principle of action is different from that in the low-pressure method. Here the water is raised to so high a temperature that it wholly overcomes the effect of gravity, and rises to the highest rooms of a building if required, the circulation through the system of pipes being more rapid as the heat of the water is greater. But there are inconveniences attending the method. If the pipes be not very strong, they will be burst by the intense pressure from within; as they will likewise if the expansion pipe be too small. If, on the contrary, this latter pipe be too large, it occasions the water to be driven up into it so violently as to leave the lower part of the coil of small pipe almost empty, and therefore liable to be burned by the heat of the fire. And if all these points be properly attended to, there is still the inconvenience resulting from the decomposition of the floating particles in the air, by the highly-heated metal of the pipes. In some cases water, instead of being heated in a coil of small pipes, passes into and through large flat boxes or chambers, whose extended surface enables the surrounding air to be heated more rapidly. The details of this chapter will enable the reader to perceive, that that part of the builder’s art which relates to the construction of the _fire-place_ rests on more scientific principles, and is more liable to change by successive discoveries and inventions, than most others. It is not simply to make a square opening by the side of a room, to have a vertical chimney or flue above that opening, and a few bars within it; it is not by such means that the object to be answered by a fire-place can be attained; some knowledge of chemistry, pneumatics, and hydraulics, is required before we can properly regulate the combustion of our fuel, the ventilation of our apartments, or effectually warm them by the ascension of hot air, the circulation of hot water, or the condensation of steam. CHAPTER VIII. THE WINDOWS AND LEAD-WORK. We must now give to our dwelling-house those conveniences which call for the services of the glazier and the plumber. These two occupations are so often combined by the same tradesman, and the two classes of operations thereby resulting are both so necessary to the finishing of the _exterior_ of a house, that we may conveniently treat of them in one chapter. Introduction of Glass-Windows.