Ventilation is closely related to cubic space, as if it is well contrived the latter may be diminished; the object being to get rid of impure air as rapidly as it is produced, and admit a sufficiency of fresh air, without causing injurious draughts to come in contact with the horses. Free ventilation is greatly promoted by the tendency of heated air to expand and ascend; so that if an exit be allowed it, towards the roof of the stable, it will escape, and cold air will rush in from below to occupy its place. If it cannot enter below, then it will from above, and cause down draughts.

The best system of ventilation is that which ensures a uniform supply of cool fresh air by night as by day without causing gusts or draughts, especially upon the legs. But this is not always easy to obtain, and particularly in stables which contain a large number of horses; small stables are more easily well ventilated.

Various plans of ventilating have been proposed, but they are all the same in principle, and are based upon the above-mentioned requirements, which may be summed up as escape for vitiated air, and introduction of fresh air without draughts. To ensure this object, it has been recognised that the outlet passages or flues should be more in number, but less, collectively, in sectional area, than the inlet ones, the flow of air into the stables not exceeding five feet per second ; so that by having a greater number of outlets than inlets, and these as far as possible from each other, the air on entrance is diffused gently, and draughts avoided.

In the case of single stables with a loft above, and where the horses stand all one way, it has been recommended to have* a nine-inch earthenware drain pipe carried in the form of a syphon through the rear wall, so as to discharge the supply vertically, at or near the ground level; while the outlet shafts (one for each three horses), which may be six-inch iron rain-spouts, should be fixed immediately over the stall partitions or bales, on the front wall, the bottoms of the pipes being level with the underside or ceiling of the stable. These pipes should be carried through the loft and the roof, and capped in such a way as to prevent rain from entering them, though not hinder-ing the escape of hot air. In double stables where the horses stand tail to tail, the same principle has been applied, but the inlets are carried from the outer wall under the stalls, discharging the air in the centre of the passage or gangway, both the front and rear walls being provided with exhaust or escape shafts.

If the windows in the stable are properly constructed and well placed, they greatly assist the ventilation, though they most frequently cause draughts, whether they are above the horses' heads or at the ends of the stable; while in bad weather they have to be closed. Indeed, it only too often happens that the horse keepers will insist upon keeping them closed in all weathers, and as the ventilation should be, to some extent, at least, independent of these men and of accident, it ought to be self-acting and beyond reach of interference. Therefore, if the outlet or air-escape shafts cannot be contrived as above indicated, a course of perforated bricks, or a row of perforated plates, should be introduced close to the ceiling, and on both sides of the stable, if thought advisable, so as to allow the hot air a ready exit.

When the loft above is intended for the reception of forage, there should be no communication between it and the stable, whereby the air from the latter may taint the food.

Where there is no loft, or stories above, then the air-escape can readily be secured by the open roof, either by a narrow, capped slit running the whole length of the ridge, by louvre boards, or by roof-windows, which can be made to open and shut. The ridge arrangement is good, as it is constant; the louvre boards, whether continuous or detached at intervals (the continuous are best), are also good; while the roof-windows are objectionable for several reasons.

The louvre boards are generally recommended, and, if continuous, should be about 18 inches in depth in a stable containing two rows of horses, and about 10 inches in single-row stables. The side boards, 12 to 9 inches broad, should overlap each other, but leaving an interval between of about 3 inches, their angle being from 45 to 60 degrees, according to the exposure of the stable.

Instead of side-boards, swing-windows are sometimes introduced, these opening at an angle from the inside of the stable; and, when it is desired to have the stable well lighted, glass is preferable. Sometimes the roof of the louvre board itself is of glass, which certainly gives abundance of light, though it makes the stable hot in summer.

Inlets for the air should be near the floor of the stable, as it tends to dry this, and also because it has a tendency to ascend towards the nostrils. It should not be allowed to enter beneath the manger, unless the current is broken or diffused previously, as it will chill the horse's legs when he is standing, and his body when lying down. This break in the current can easily be effected by laying the outer course of perforated bricks at either a lower or a higher level than the inner one. It is best, however, to have the air enter - if it must enter through the front (or manger) wall - between the stalls, as then it will not impinge directly on the horses. But there is no absolute necessity for having the air enter only on one side, or even in any special part of the stable, so long as there is an abundance without draughts.