A further development of the hot-water system then came about. In this device the radiators were made to contain water, but the heat was circulated through the pipes by means of steam. This steam was poured over the surface of the water in the radiator and transferred its heat to it. According to the quantity of steam poured over the water, the latter could be heated to various temperatures. Of course the water in the radiator was the medium for distributing the heat outward from the radiator itself. Still another improvement was made upon the hot-water system by introducing the principle of the closed expansion tank. In the ordinary system the water is allowed to expand at the top through an expansion tank, so that the actual pressure on the water of the system is atmospheric. Under this pressure the temperature of the water cannot be raised to more than 212 degrees Fahrenheit, for beyond this it boils and changes to steam. However, in the closed-tank system a so-called heat-generator is added on the line leading to the expansion tank, which, by means of a column of mercury, is capable of adding 10 pounds more pressure than the atmosphere to the water in the system, and thus raising the boiling-point to about 240 degrees. This generator is so designed, however, that, although it adds this greater pressure to the water, yet the natural expansion of the water in the system is permitted through it in case of emergency. By permitting the raising of the temperature of the water, the size of radiators can be cut down 50 per cent, which, of course, reduces the quantity of water needed and permits a quicker heating of the system when the fire is started. Thus a saving of fuel is accomplished and the disadvantage of the ordinary hot-water system is eliminated; namely, the long time required to get hot water in the radiators after the fire is started in the morning from its banked condition of the previous night. [Illustration: Pipeless Furnace] However, the genius of the inventor was not at rest on the problem of warm-air heating, for he discovered that he could abolish the flues, which he once thought were essential, and use but one register and one flue. This is called the pipeless furnace. A register is employed which has an outer and inner section. The outer section permits the cold air from the house to pass down through it and over the drum of the furnace. The inner section of the register permits this hot air to escape upward and through the house by natural distribution. Thus the hot air rises from, and the cool air settles back into, the furnace without utilizing flues. The circulation of this system was found to be superior to the older method as ordinarily installed, and very much cheaper to install. In fact, it is the cheapest of all systems of heating. It is especially adapted to the small, low-cost house. [Illustration: Hot Water Heating—Boiler in Dining-Room]