It is quite generally agreed that the velocity of the entering air should not exceed four to six feet per second unless it can be introduced in such a position as to make an insensible current. The table which has just been given, while only approximately correct, gives a very fair idea of the sensations produced by air-currents of different velocities and pressures, and is useful in fixing limiting values.

The most effective location for the air-inlet is probably in or near the ceiling of a room, although authorities differ much in this respect. The advantages of introducing warm air at or near the top of the room are: first, the warmer air tends to rise and hence spreads uniformly under the ceiling; second, it gradually displaces other air, and the room becomes filled with pure air without sensible currents or draughts; third, the cooler air sinks to the bottom and can be taken off by a ventilatingshaft. So far as the system introduces air at the top of a room it is a forced distribution, and produces better results than other methods. When the inlet is placed in the floor or near the bottom part of the walls it is a receptacle for dust from the room, and a lodging- and breeding-place for microbe organisms. In the ventilation of large buildings the inlets can usually be located in the ceiling, especially if the lighting be done by electricity or in some manner not affected by air-currents.

Some experiments were made by Mr. Warren R. Briggs, of Bridgeport, Conn., on the subject of the proper method of introducing pure air into rooms and the best location for the inlet and outlet. The experiments were conducted with a model having about one sixth of the capacity of a schoolroom to which the perfected system was to be applied. The movements of the air in the model of the building were made visible by mingling the inflowing air stream with smoke, which rendered all the changes undergone by it in its passage apparent to the eye.

The results of the experiments are shown graphically in the six sketches. Figs. 15 to 20. In each case the distribution of the fresh air is indicated by the curved lines of shading. A study of these sketches is very suggestive, as it indicates the best results when the inlet is on the side near the top, and the outlet is in the bottom and near the centre of the room. The

tendency of the entering air to form air-currents or draughts, which in some instances tend to pass out without perfect diffusion, is well shown. This tendency is less as the velocity of the entering air is reduced, and we probably get nearly perfect diffusion in every case where the outlet is well below that of the inlet, provided the velocity of the entering air is small -less than 4 feet per second.

35. The Outlet for Air.-The outlet for air should be as near the bottom of a room as possible, and it should be connected with a flue of ample size maintained at a temperature higher than that of the surrounding air, unless forced circula

tion is in use, in which case the excess of pressure in a room will produce the required circulation. If the temperature in a room is higher than that of the surrounding air, and if the flue leading to the outside air can be kept from cooling and is of ample size and well proportioned, the amount of air which will be discharged will be given quite accurately by the tables referred to. These conditions should lead us to locate ventflues on the inside walls of a house or building, and where they will be kept as warm as possible by the surrounding bodies. If for any reason the temperature in the flue becomes lower than that of the surrounding air the current will move in a reverse direction, and the ventilation system will be obstructed.

The conditions as to size of the outlet register are the same as those for the inlet; the register should be of ample size, the opening should be gradually contracted into the flue, and every precaution should be taken to prevent friction losses.

36. Ventilation-flues.-The size of ventilation-flue will depend to a great extent upon the character of system adopted, but will in all cases be computed as previously explained. A practical system of ventilation generally is intimately connected with a system of heating, and the various problems relating to the size and construction of ventilating ducts will be considered later. In general the ducts should be of such an area as not to require a high velocity, since friction and eddies are to a great extent due to this cause.

The size of the ventilating duct can be computed, knowing its rise, length, and the difference of temperature by dividing the total amount to be discharged

by the amount flowing through one square foot of area of the

Aue under the same conditions.

In introducing heated air into a room, it is very much better to bring in a large volume heated but slightly above the required temperature of the room rather than a small volume. at an excessively high temperature. If the temperature of the air be brought in 25 degrees above that of the air in the room, the discharge in a flue one square foot in area would be in cubic feet per minute, 171 for a height of 10 feet, 271 for a height of 25 feet, 342 for a height of 40 feet. By referring to the table, Article 33, the discharge for any condition can be readily determined.

As the difference of temperature of the air in the room and outside may usually be taken as 20°, the velocity in feet per minute for heights corresponding to the distance of floor to roof in a building of 3 stories would be about as follows: 1st floor, 306; 2d floor, 242; attic or top floor, 188,-or about 5, 4, and 3 feet per second. For air discharged, the order of the velocities would be reversed on the particular floors. The area of the flue would be found by dividing the total air required per second by these numbers.

The general arrangement for heating the air and introducing it into a room is shown in Fig. 21. In this case the cold air is drawn in at D and delivered into the chamber C, whence it passes through the heater, thence into the flue, entering the room at the register B. The vitiated air enters the ventilating flue at E.

37. Summary of Problems of Ventilation.-From the foregoing considerations it is to be noted that the practical problems of ventilation require the introduction, first, of thirty or more cubic feet of air per minute for each occupant of the room, and in addition sufficient air to provide perfect combustion for gas-jets, candles, etc., which are discharging the products of combustion directly into the room. Second, the problem requires the fresh air to be introduced in such a manner as to make no sensible air-currents, and to be in such quantities as to keep the standard of contamination below a certain amount. This problem can be solved by either, first, moving the air by heat, in which case the motive force is very feeble and likely to be counteracted by winds and adverse conditions; second, by moving the air by fans or blowers, in which case