Warming by open grates or closed stoves is specially applicable to the rooms of private houses; warming by hot air or steam, or hot water, is chiefly used for large buildings. It is quite possible that these methods will be applied at some future time on a large scale to the warming of private houses. In some large towns of the United States this has been already done, blocks of a hundred or more houses being warmed from the same centre, by the same system. But apart from such a central system, hot air and hot water lend themselves to the heating of houses on what may be called the _Whole House System_ (page 156). We have mentioned in the last chapter some methods of doing this, and shall now describe others. =Hot-water Pipes= are probably the best means of carrying heat to various parts of a large house, and hot water is more thoroughly under control and less dangerous than either hot air or steam. There are _two systems_ of heating by hot water. In the first, which we may call the =low pressure system=, there is a boiler from which water circulates through pipes to every part of the building, and as it cools down returns again to the boiler. At the highest points of the pipes, outlets are provided for air. In this system the water is not heated above 200° Fahr., and there is consequently no great pressure on the pipes. In the =high pressure system= (Perkin’s patent), the pipes have an internal diameter of about 1∕2 an inch, and have thick walls made of two pieces of welded iron. There is no boiler, but one portion of the tube passes through the fire and the water is heated to 300-350° Fahr., thus subjecting the pipes to great pressure. In dwelling-houses with the low pressure system, for every 1,000 cubic feet of space to be warmed to 50°, 12 feet of 4-inch pipe should be given; with Perkin’s pipes, probably about two-thirds of this will suffice. =Steam Distributed by Pipes= may be employed instead of hot water. This method has been used in factories in which there is a surplus supply of steam. =Warming by Hot Air= is only applicable on a large scale, and should only be used in association with a system of ventilation by propulsion (page 153), in which the temperature, humidity, and freedom from dust of the entering air are carefully regulated. =Warming by Electricity= both for cooking food and for warming rooms has a large future, but in most districts the supply of electricity is not hitherto sufficiently cheap to be used for these purposes. By its means the atmosphere will be prevented from becoming impure, labour will be reduced, and life rendered more pleasant.