There are always the doors and windows, which may be thrown wide open at intervals, and in many houses there are fireplaces. If a window be opened at the bottom at one side of a room, and another be opened at the top on an opposite side, a current of air will be established from the first window, passing through the room and out at the second. This plan will do very well in warm weather when the temperature outside is about the same as that of the room, but it would be impracticable in cold weather. Then we may resort to the very simple plan of placing a board about eight or ten inches wide across the window at the bottom and inside of the sash. Then when the lower half of the window is raised, a space is left between the upper and lower sashes, through which the air passes freely as it enters, and, being sent into the room in an upward direction, causes no draft. The board is for the purpose of closing the window below, and should fit quite close to the sash. Fireplaces are good, though not perfect, ventilators. Then there are the preventive measures, such as burning the gas or lamp low at night, avoiding oil- and gas-stoves, etc.; the latter are the worst possible means of heating rooms, for not only do they draw oxygen for burning from the air, but they give out the polluting carbonic acid and other products of combustion, which in a coal- or wood-stove go up the chimney. A well-ventilated room should have an inflow of warm, pure air, and a means for the removal of the same after it has been used, the current being so controlled that, although the air is kept in motion, there is no perceptible draft. The plan for the heating and ventilation of the Johns Hopkins Hospital, Baltimore, Maryland, is a most admirable one. Air from out of doors is conveyed by a flue into a chamber in the wall, in which are coils of pipe filled with hot water. The air in passing over these becomes warm, and, rising, passes into the room to be heated through a register. On the opposite side of the room is a chimney-like flue, running to the top of the building and containing two registers, by the opening and closing of which the movements of the air in the room can be controlled. The temperature is maintained by the temperature of the water in the pipes, and the rapidity of the flow.[14] The ventilation by this method of heating is the most perfect known to the author, who has lived for two years in a building thus supplied with warmth and fresh air. The rooms were invariably comfortable as to temperature, and the air as invariably sweet and pure. MILK