pends principally upon the degree of pressure at which the jets issue from the nozzle. All jets are apt to be noisy, compressed air being least so. They can be used either for extraction or propulsion; in the latter case the steam jet will moisten the incoming air considerably, which may be either an advantage or the reverse.

Arrangements for Artificial Ventilation. There are certain points that require attention in all arrangements for artificial ventilation:

(1) The point of intake for the fresh air should be selected principally for the purity of the air obtained, and, as a general rule, the purest air will be found at a height of 3 to 4.5 meters (10 to 15 feet) from the ground.

(2) The air may require cleansing or filtering. With this object in view it may be made to impinge on a sheet of still water or, better, a film of glycerin, which retains impurities better and does not evaporate so readily. The air may be filtered through coarse cloth or cotton, the latter being most effective, but requires frequent renewal. A thickness of about 1.4 decimeters (6 inches) may be employed. The air may also be purified by washing it with a spray of water or passing it through a wire screen over which a fine stream of water is running. This adds moisture to the air and may require supervision, as hot, moist air produces languor.

(3) The temperature of the incoming air should be regulated. It may be cooled in summer by passing over ice, or, if the water spray is used, this may be cooled by ice, which is an effective method of reducing the temperature. An apparatus devised by Professor Gates, of Washington, for the cooling of rooms in summer, can be more cheaply operated than a coal stove in winter. It is simply a tall cylinder of galvanized iron resting in a large basin or pan, and connecting at the top with the ordinary stove pipe or with a tube leading out of a window. In the top of the cylinder's interior is a perforated tubular ring, and on a cock being turned on this ring an artificial shower is produced inside the cylinder. The water thus flowing down the sides takes on a rapid spiral motion

which sucks the air down the cylinder at a rapid rate; the fine spray inside cooling the air, reducing its humidity to normal, and purifying it of all dust and odor. The water collects in a basin below, from which it is drained off, the cool air escaping through openings just above the water surface of the basin. In some experiments made with this apparatus the temperature of the air on entering the cooling cylinder was observed to be 33° C., while it was 20° C. on taking its exit at the bottom. Recently "liquid air" has been introduced as a means of cooling the air, and has been applied with satisfaction in the ventilation of theaters.

The incoming air may be warmed by passing over or through a heating apparatus, such as hot-water or steam pipes. This is the method now commonly employed in the ventilation of large buildings. The whole of the air-supply, in a scheme. of artificial ventilation, ought to be admitted at the required temperature to the chamber to be ventilated, and no attempt should be made to warm the room by superheated air. As a rule, the fresh air should be warmed as near as possible to the temperature desired in the room. In large buildings, consisting of many rooms, the scheme of ventilation requires separate and perhaps different arrangements in different portions. In other buildings, such as churches and theaters, a single central scheme is preferable.

(4) The channels through which the air is conducted should be so arranged as to be easily cleansed; this is especially necessary in propulsion methods, and inattention to this point has in many instances brought the method into disrepute. Where the air has been previously filtered there will, of course, be less deposit of dust on the sides of the inlet shafts. Extraction shafts also require to be kept clean.

The inlet and outlet openings must be so placed that a thorough aëration of the room is possible without the production of draughts. Different arrangements are required for the inlet and outlet openings for winter and

summer. In summer the cool air is brought in at a height of from 2 to 3 meters above the floor in such a manner that it is conducted toward the ceiling. From this point it sinks gradually, and when warmed rises and takes its exit at the opposite side of the room near the ceiling. In winter the air takes its exit near the floor.

The system of ventilation should be so constructed that it can be regulated to meet all reasonable requirements as to quantity of air furnished and the temperature that may be demanded by atmospheric conditions.

Removal of Dust.-The removal of dust in factories requires special arrangements. The circulation of air in closed rooms is insufficient to keep the dust from settling, and, especially in factories, special arrangements for its removal are required. Where large quantities of dust are produced, as in certain factories, it is often possible to apply a strong air current near the source of the dust in order to aspirate it, as in the case of circular saws and grindstones (Figs. 19, 20, 21). Where the dust cannot be satisfactorily removed by such a method it is often possible to modify the manufacturing process so as to prevent excessive dust formation. The use of a spray of water so as to render the material damp and thus prevent, in large part, the formation of dust will aid in keeping the air pure.

In houses and hospitals dust is to be prevented as much as possible. If dust has formed, it tends to settle upon horizontal surfaces, and should be removed with damp cloths.

Comparison of Extraction and Propulsion Methods.The extraction method is less costly and utilizes the naturally high temperature of the vitiated air. Its disadvantages are that the source of the incoming air is not under control, and, consequently, impure air may be admitted and there is greater liability to draught. In the propulsion method the inlets are entirely under control if properly arranged, and the purity of the air can be assured, as well as its suitable temperature and velocity,

so as to avoid draughts. A proper diffusion of the incoming air throughout the room is more easily effected in the propulsion than in the extraction method. The disadvantage of propulsion is its greater cost. A combination of the two methods is frequently employed, and meets all the requirements.

Comparison of Natural and Artificial Methods.—(1) Natural ventilation is rarely sufficient, and usually re

FIG. 19.-Apparatus for removing dust in manufacturing establishments: a, inlet to exhaust shaft; b, valve regulating spray; c, water-supply pipe; d, outlet of exhaust shaft.

quires to be supplemented. (2) For dwellings, extraction by heat by means of open fireplaces and chimneys is generally sufficient. It is automatic and requires no special attention, but it is not a perfect system of ventilation. (3) For large halls, churches, theaters, and schools artificial ventilation is necessary. In buildings of this character mechanical methods have a decided advantage over

FIG. 20.-Apparatus for removing dust in manufacturing establishments: a, emery weeel; b, hood over emery wheel; c, exhaust shaft.

natural ventilation, not only in the greater purity of the air, but in the more equable temperature attainable.

FIG. 21.-Apparatus for removing dust in manufacturing establishments:

a, blower; b, dust-collecting chamber; c, water-sprays.