I have already described the Smead system, which is so well known in the United States, and also a characteristic continental system; it will now be of interest to see how the same problem has been treated by a British engineer, Mr. Key, and to note the radical differences between the systems. One of the objections raised to the Smead system by Drs. Drysdale and Hayward, and also by others, was that the direction of the currents of heated air in the rooms was such as to cause the vitiated air to be breathed over again. The objection has not perhaps much force when gas is not being burnt, for it is the practice of the Smead Company in the United States to change the air in schoolroom- about six times per hour, and to provide from 1500 to 1800 cubic feet per head.

The Key system is what is known as the plenum method of warming and ventilation. The inventor early realized that the external air is so charged with soot, dust, and fog, that it is absolutely necessary to clean it thoroughly before it is admitted into a building; he also found that a dry cloth screen is of little use for the purpose, as it allows the particles to pass through it after a time, and becomes coated with dirt, which in itself may become a source of danger. He thus describes his system. " The air-supply for a building is drawn, by means of an air-propeller or fan, from a point where it is of undoubted purity, and furthest from any possibility of contamination. The entering air passes through an outer warming coil, and then through the air-filtering, air-washing, and humidifying screen. It is then warmed by coming into contact with coils, clustered in batteries within the air-warming chamber. The air passing through this chamber can be instantly reduced in temperature by admitting filtered cold air. through the by-pass doors provided for the purpose, the warm and cold air mixing while passing through the air-propeller. The air is then propelled into the main air-ducts, from which it passes into flues leading to each room. Secondary air-warming coils are placed at the base of each flue, so that the aircto each room may be warmed to any desired temperature while passing through them, and independently of the others.

The volume of air admitted to each room is directed towards the ceiling, and can be regulated, or shut off altogether. It enters under a slight pressure, and is therefore continuously forcing out the air previously within the room. This may be done at a rate to renew the air of a room from 6 to 15 times per hour, and without experiencing [it is said] the slightest draught. The outgoing air passes off at the floor-level, and is led to roof ventilators, where the outlet air-valves are so constructed as to place the whole air within the building under a slight pressure of about four ounces per square foot in excess of the outside atmospheric pressure at the time. Whether there he no air movement outside, or whether it be blowing a gale, the outgoing air [we are told] flows in a continuous stream, unaffected by calms or gales."

The apparatus for filtering and washing the air consists of several thousand cords of suitable material, stretched from a beam near the ceiling to another near the floor of the air-chamber. When finished, the screen has the appear-ance of coarse cloth stretched across the apartment. The cords are placed so close that they touch each other; copper wires are laced through the vertical cords in horizontal rows, and being drawn tight, give the screen a flat surface. The rough fibrous nature of the material breaks up the entering air into very minute streams, which pass through equally all over its surface. These screens may, if desired, be formed double, in order to give an extra cleansing or scrubbing surface. The screen is kept moist by water trickling down each cord; and at regular intervals of more or less frequency, an automatic flushing-tank discharges a considerable volume of water down the screen, to remove loose matter which may have collected, and to thoroughly wet the whole surface,

Reference has already been made to the necessity for very careful attention to the humidity of the air used for warming, and it will be seen that the temperature of the incoming air, in this system, is first raised, then washed and humidified by the screen, and afterwards further heated by the local coils. It is warmed to a temperature of about 57° F. in passing through the primary air-warming chamber, and no portion of the air is raised above this temperature by contact with the heated coils. Either hot water or steam may be used in the pipes as the heating medium, but the inventor appears to prefer to use steam, as, of course, a somewhat smaller exposed surface will suffice, while at the same time none of the air is "burnt". One of the minor defects of the system appears to me to be the difficulty or impossibility of regulating the degree of humidity given to the air, as the quantity of water passing over the screen is practically constant

With regard to the general system of admitting the fresh air at the ceiling, and discharging the foul air at the floor-level, it may be said that the wisdom of tins course depends largely upon the method of illumination. If coal-gas is burnt for illumination in the ordinary way freely in the air, the heated products of combustion naturally rise to the ceiling, and should, in my opinion, be removed thence. Where the outlet-holes are at the floor-level, the products of combustion (including carbonic acid gas, C02) must pass down and be breathed. For this reason, I am opposed to the downward system of warming and ventilation in such cases. Of course, if electric lighting be employed, no foul gases are produced by the lighting, and the objection would not hold; there is, how-ever, a certain very small amount of heat produced by the lamps, which would have some little effect in producing or assisting upward currents of air.