The true theory of ventilation is to obtain a perpetual and sufficient change of air without sensible draught. The following simple plan, as I have proved by years of experience, perfectly fulfills these requirements, and leaves nothing to be desired. The _Scientific American_ endorses the plan, and places it above many, in fact most of the elaborate and expensive devices. The eminent Dr. B. W. Richardson, of London, also, is on record in favor of the plan, and it is already in use in thousands of homes in this country. A three-inch strip placed beneath the lower sash of each window has the effect to “mismatch” the sashes, causing them to overlap each other in the middle. The stream of air thus admitted is thrown directly upward, and slowly mixes with the heated air in the upper part of the room. As several windows in each room are thus provided, the vitiated air is constantly passing out at one or another of the ventilators. The strip being perfectly fitted or listed, no air can enter at the sill, and all can be so nicely finished as in no manner to mar the appearance of the most elegant drawing-room. A dwelling thus ventilated will never smell “close” to the most sensitive nose upon re-entering, even after a prolonged stay in the open air——a test that would condemn, as unfit for occupancy, ninety in the hundred sitting and sleeping rooms, as well as churches, halls, etc., the world over. The purity of the air is by no means measured by the temperature. Cold air is often very impure by reason of stagnation (as stagnant water), or the exhalations from the lungs, etc., while, on the other hand, the temperature may be maintained at 70° F., or upwards, without fatally lowering its quality, if a sufficient and perpetual change is going on between the outdoor and indoor air.