It will not be questioned that a supply of pure air is absolutely essential to health, and accordingly the subject of ventilation has always attracted a large amount of attention from sanitary authorities. In theory nothing can be more simple; it is only necessary to provide openings through which pure air may pass in at one point, and other openings, in a different position, out of which the contaminated air may escape. This is, undoubtedly, the true principle of ventilation, but in practice it has been found extremely difficult to attain the results which are desired. The pure air is commonly found to enter with sufficient rapidity, and often in sufficient quantity, not only at the point through which it was intended to enter, but also at the opening which was intended for the escape of the contaminated air. The down-draught, as it is called, is the great trouble of the sanitary engineer, and it has not up to the present time been found possible to avoid this and to create an upward current with perfect certainty and regularity except by the use of machinery, or the employment of heat to rarefy the air at the intended point of exit, so that the air contained in the building may be induced to rise and escape as required.
Some interesting experiments were performed by Veterinary Major Fred Smith of the Army Veterinary Department, and described by him in his work on Veterinary Hygiene. The object was to ascertain the direction of currents, after entering a building by the means of windows, tubes, shafts, perforated bricks, or holes in various parts of the walls. The first thing which was noticed was the diminution in the rate of motion of a current of air in the act of passing through a shaft or tube, owing to the friction against the sides of the passage. It follows, therefore, that the loss of motion in the air will be considerably less in a wide than in a narrow passage. A further cause of diminution of velocity and interference with escape of air exists in the bends or angles in the passage, and it is important to remember that in such bends accumulation of dust is inevitable, and that when bends are unavoidable some method must be devised for the purpose of keeping them clean.
A very common device for ventilating a building is that of the shaft divided into two by a diaphragm running down the centre. By this method it is presumed that one side of the shaft will act as an inlet, and the other side as the outlet; but in practice the operation is by no means always satisfactory. The same may be said of a somewhat similar arrangement, the double tube: a large one for the outside, and a small one passing through it. In this plan the larger tube is intended to act as an inlet, and the smaller one as the outlet. But it is very commonly found that the result is a down-draught always in full action, while the outlet either has the opposite to the intended effect, contributing to the down-draught, or otherwise does not act at all.
It must be obvious that the force and direction of the wind must always be powerful factors in ventilation, and one difficulty which is not easily overcome arises from the frequent changes which they undergo, at one time a superabundance of air being driven forcibly into the building, while at another a scarcely calculable quantity will pass in. To meet this difficulty, to some extent, the author of Veterinary Hygiene is in favour of openings being made on opposite sides of the building; and he insists that to get the full benefit of such an arrangement the buildings must not be more than from 25 feet to 30 feet wide. He found that a current entering through an inlet will cause the air in the stable to set in towards it in a direction more or less at right angles; and if the velocity of the incoming current is great, it may pass out again at the opposite opening before it has properly mixed with the air in the stable. The results of the experiments made by the author of Veterinary Hygiene will be best understood with the aid of the accompanying diagrams, which are taken from his work. In the first illustration the wind is supposed to enter at a window which is opened to windward (fig. 464). Soon after entering, the current is described as spreading out fan-shaped and passing towards the ground; and in the case of a powerful current it may be measured 18 or more feet from the point of entry, but under ordinary conditions its speed rapidly decreases owing to the pressure of the air in the stable at about 6 or 8 feet from the inlet. The current is further described as striking the ground on the opposite side of the stable, much of it escaping by the leeward side of the ridge, or by the opening by which it entered. The windward side of the ridge opening is also presumed to act as an inlet. In the next illustration (fig. 465) the windows on opposite sides of the building are shown open. With this arrangement the air which rushes in at the windward side was observed to proceed very much in the same way as in the first instance. Some of it, however, is driven straight across to the leeward window and escapes at once.
Fig. 464. - Direction taken by Air-currents with the Windward Windows open.
Fig. 465. - Direction taken by Air-currents with Windward and Leeward Windows open.
Sometimes the movements indicated in fig. 465 are disturbed by a backward current suddenly coming in through the outlet window, meeting the current which comes from the windward window (fig. 466). The two currents then spread out towards the centre of the stable, strike the ground, and then rise to escape at one side of the ridge. This condition is one which, as a matter of course, depends on a change in the direction of the wind, and is, therefore, only temporary.