A different set of movements occurs when the opposite windows are half-open in such a way (fig. 467) that the current of air is directed upwards. By this arrangement the entering air is delayed sufficiently long to allow it to mix with the air in the stable before it escapes from the leeward window. It was observed that if the windward window remains half-open, and the leeward one fully open, the air escapes from the stables without mixing properly.
A further change was noticed when the windward windows were half-open and the leeward entirely closed. The current passed upwards to the ceiling, and then descended and mixed with the stable air - the ridge in this case constituted the outlet.
When all the windows were closed, and the door opened, the ridge appeared to act as a regular outlet; with both the doors and windows shut the ridge openings acted as inlets and outlets, as shown in fig. 468.
This summary of the description given by Major Fred. Smith will convey to the reader a fair idea of the influence which the opening of doors and windows can be made to exercise upon the distribution of air throughout a stable. The great value of these observations lies in the fact that they refer to the very simple appliances which are at everyone's command, consisting merely of ordinary doors and swing windows, placed opposite to each other in walls not more than 30 feet apart.
Fig. 466. - Double Currents from opposite Windows.
Fig. 467. - Direction taken by Air-currents when opposite Windows are half-open.
Fig. 468. - Direction taken by Air-currents when Doors and Windows are closed.
Writers on ventilation describe artificial ventilation as distinct from the "natural" ventilation obtained by ordinary appliances such as doors, windows, and holes in buildings. Artificial ventilation may be arranged to operate in one of two ways, namely, (l) by "extraction" of the air which is already in the building, and (2) by "propulsion", which consists in the driving out of contaminated air by the forcible introduction of fresh air. Extraction is effected by heat, by steam-jet, or by fan or screw. The most simple instance of "extraction" by heat is that of the common fireplace, with its open chimney, in which the upward current is in proportion to the amount of heat and the area of the chimney.
Dr. Parkes refers to a room which he frequently examined where the area of the chimney was 1.5 square foot. There was no down-draught, but a constant upward current of 4 feet per second; the discharge per second was then 6 cubic feet, or 21,600 cubic feet per hour. The capacity of the room was 2000 cubic feet, so that a quantity equal to the total air in the room passed up the chimney nearly eleven times per hour. Notwithstanding this, the room became close when shut up with two or three persons. The explanation given is, that when the windows were shut the fire was chiefly fed with air which entered below the doors, and, flowing near the ground to the chimney, was never properly diffused through the room. It was found that the current near the around moved from 1'6 foot to 2'6 feet per second, and chilled the feet. A few feet above the ground no movement could be discovered. No better example than this could be given of the great importance of arranging for the proper entrance and distribution of air as well as for its exit.
When a fire is lighted, all places in the room through which air can pass act as inlets, and as the necessary result currents in various directions come from places which were meant to be outlets, causing what are so very much dreaded by people in general, so-called draughts. The common remedy for this state of things is the blocking up of all the cracks in doors and windows which can be reached, and the plugging of any ventilating tubes or shafts by the aid of dusters or any other material at hand. In a stable a mass of hay forms a convenient plug for any hole through which the air passes too freely for the comfort of the persons employed about the stable. In a room so treated, it is noticed that when all the openings through which air can enter are plugged, the chimney itself becomes an inlet at intervals, and consequently sudden rushes of downward currents occur, bringing with them a quantity of smoke; but when the inlet of air is properly regulated and provided for, the open fireplace with its chimney is undoubtedly a very useful method of ventilation. It may be urged that the method is not generally applied to a stable, but in the case of new constructions there certainly is no reason why it should not be, provided that a suitable wire screen is placed around the hearth to prevent any risk of straw, etc, catching fire.
Extraction of air by a steam jet requires apparatus not in common use, and is not likely to be employed for the ventilation of stables.
Extraction by means of fans is a method which has been employed successfully in the ventilation of mines, and to a smaller degree in buildings, but the plan involves cumbersome machinery, and can hardly be called a practical method of ventilation for stable use. The place of the fan may be, to some extent, supplied by means of different forms of cowls, the chief objection to which is their uncertainty, owing to changes in the direction of the wind, and in perfectly calm weather the absence of any currents in the outside air. These appliances, however, form part of the apparatus used in so-called "natural" ventilation.
Ventilation by propulsion, although a powerful method of delivering a quantity of air, is not one which is likely to be generally used in stables. According to Dr. Parkes, the plan is an old one, invented indeed by Desaguiliers in 1734. The machinery consisted of a fan or wheel, enclosed in a box. The air passed in at the centre, and was driven by the vanes of the fan into a conduit leading from the box to the building to be ventilated. The principle of this system, which is now generally known as the "Plenum" system, is that of pressure from behind, the external air being forced in at a pressure proportionate to the speed of the revolution of the fan, thus driving out the fouler air through the openings provided for the purpose. Where expense is no object the plan is no doubt an effective one, as air can be passed through water and thus washed, or through heated tubes and thus raised to any temperature which may be desired, but in practice its use has hitherto been confined to very large establishments, town-halls, hospitals, etc. Where electric current is available, an electric fan, which can be installed at a trifling cost, is the simplest method of applying the system.
It must be admitted that all methods of ventilation which necessitate the use of special apparatus are encumbered by the objections that they are costly and in various ways inconvenient. Among others is the very important one, that skilled attendance is necessary.
For practical purposes the method of ventilation which has been described, by the aid of openings at opposite sides of the building and at the ridge, is the most simple, and, if properly arranged, the most effectual.
A certain quantity of fresh air is absolutely indispensable for the maintenance of life. A horse requires something over 15,000 cubic feet of air per hour. But the question is not one of quantity only; there can be no doubt at all that the exact amount of air which a horse requires may be fully provided and yet be in such a condition as to destroy the animal in a short time.
The difficulty of ventilating a stable is increased by the objection which the groom entertains to a current of cool air, which, however pure, will make his horse's coat rough, or cause the warmly-clothed animals, which have been habitually kept in hot stables, to shiver. This is a fact which the groom will demonstrate without any difficulty, to ensure conviction in the mind of his master, by opening a window behind one of his horses and causing the animal to shiver forthwith as soon as the unaccustomed cool current touches its skin. After this demonstration, with which stablemen are perfectly familiar, the question is settled at once, without any further argument, and the owner of the animal, if not convinced, is at least silenced.
The demonstration, although utterly fallacious, contains a valuable suggestion, to the effect that the air of the stable should, by some means, be properly regulated to a moderate temperature, so that the horses should not be subjected to either hot or cold currents of air.
If open fire-places with warm-air chambers are not provided, perhaps the most satisfactory way is to heat the stables (or the air entering them) by means of a low-pressure hot-water heating apparatus.