Most people do not imagine that one set of apertures are required to carry away the foul, and another to admit the pure air. Even those who know that one set can not answer both purposes in a perfect manner, are apt to disregard any provision for admitting fresh air. They say there is no fear but sufficient will find its way in somehow, and the bottom of the door is usually pointed to as a very good inlet. It is clear enough that while air is going out, some also must be coming in; and that if none go in, little or none can go out. To make an outlet without any inlet, betrays ignorance of the circumstances which produce motion in the air. To leave the inlet to chance, is just as much as to say that it is of no consequence in what direction the fresh air is admitted, or whether any be admitted. The outlets may also serve as inlets; but then, they must be much larger than I have mentioned, and the stable, without having purer air, must be cool, or cold. When the external atmosphere is colder than that in the stable, it enters at the bottom of the door, or it passes through the lowest apertures to supply and fill the place of that which is escaping from the high apertures.

If there be no low openings the cooler air will enter from above; it will form a current inward at the sides, while the warmer air forms another current, setting outward at the centre of each aperture. But when the upper apertures are of small size, this will not take place till the air inside becomes very warm or hot.

The stables at the Veterinary College are all single-headed. Each stall has an aperture at top of the head wall for carrying off the foul air, and in the back wall there is another of the same size, level with the ground, for admitting pure air. These are covered with iron-grating to exclude vermin. This, I think, is not the best place to have these inletting apertures. In order to reach the nostrils, or head of the stall, where the impure air is rising upward, the fresh air must pass over the horse's heels while he is standing, and over a great part of his body while lying. The same thing l._ppens when it passes from the bottom of the door. A current of cold air is established, and constantly flowing from the point where it enters, to the point where it escapes, and the horse, or some part of him, stands in its path. Possibly a current so small and so feeble may do no harm, but possibly also it may have something to do in the production of cold legs, cracked heels, or an attack of inflammation. If it have any effect it can not be of a beneficial tendency, and ought therefore to be prevented if it can be prevented. It is easy to break the current and diffuse the cold air over the stable, by placing a board or some other obstacle opposite the inlet-ting apertures.

It would be better, however, if they could be placed nearer the points where the air is wanted.

In Mr. Lyon's stables (Fig. 7) there are no apertures purposely contrived for admitting fresh air. The windows serve both as outlets and as inlets. They are very large. While the warm and impure air is ascending the sides of the tunnel, the external air is descending the centre of the same passage and spreading over all the stable. This keeps it cool, cooler than would be proper where a fine coat is of more importance Still, by lowering the windows these stables can be kept very comfortable, and without rendering the air unwholesome. From the manner in which they are arranged, low apertures can not be obtained except to four stalls, without considerable expense, and I am not sure that they would be a great improvement though they were introduced.

Admitting that it is better for the sake of warmth to have small outlets with corresponding inlets, than to have large outlets and no inlets, I think the inlets ought to be placed near the horse's nostrils. To keep him warm, the air which surrounds his body should be warm and stagnant, or at least as warm and still as ventilation will permit. When the fresh air enters at some distance, it must traverse the stable to reach the place where it is consumed, and in its passage it cools the stable and plays upon some part of the horse. By admitting the fresh air at the head wall, below the manger, or near the ground, the current would be short; it would not be intercepted by the horse, and it would not cool the air which surrounds his body, and keeps him warm. A stable free at both ends, whether single or double, might have a wooden tube running below all the mangers, and at each extremity open to the external air. As it passed through each stall, a number of small perforations, widely spread and sufficient to admit the air, would be better than a single aperture.

If the stable were not very long, perhaps it might be sufficient to have only one end of the tube open; and whether open at one end or at both, the extremity should be turned downward or defended by a cap, to prevent the wind from blowing into it. I do not think that the air would ever enter with such force as to cool the horse's head or his legs. But as the plan has not been tried, whoever thinks well of it had better put it to experiment on a small scale. When the stable abuts against other buildings, this is the only mode by which fresh air can be brought to the head of the stall, without passing over the horse. When the head wall is free, an aperture can be made right through it; but this, though it might be better than having it placed opposite the horse's heels, would be objectionable. The air might come in too strongly, and blow upon the head when the horse is lying. The small sievelike perforations spread over a considerable surface, the whole forming a space equal to about six inches square, would render a current upon the head almost impossible. The only use of low apertures is to admit fresh air. In former times, it was supposed that they were necessary for taking out the carbonic acid gas formed during respiration.

It was found that this gas is much heavier than common air, and it was imagined that it fell to the ground, like water when dropped among oil. But it is now known that, though heavier, the gas unites with the atmosphere, or gravitates in very small quantities, and only till the air can absorb it.

When the floor of the stable is bad, retaining the urine and then rejecting it by evaporation, the inlets and the outlets require to be much larger than I have mentioned. A low roof also renders large apertures very necessary.

Objections Urged Against Ventilation

These, as I have already hinted, often have their origin in ignorance, which attempts ventilation without knowing its intention or the mode of producing it; and in indifference, which thinks it does well while it follows as others have led. The cost of ventilating a stable is very trifling, yet some are so awkwardly arranged that the process may demand more than the owner is willing to give. It is the most foolish of all objections; the evils produced by bad air may be attended with more loss in six months than would pay the cost of ventilating the stables six times. Even where there is no actual disease, the horses, if doing work, require more corn to maintain their condition than those who have more air.

The cold currents of a ventilated stable, to which people bo often object, are injurious only when the apertures are too large or improperly placed. If there be a large aperture behind the horse's heels, and another above his head, the cold air must pass over him, and in force proportioned to its vol-ume. But this is easily avoided, either by having a number of very small apertures, or by placing the outlets and the inlets in such relation to each other, that the horse can not 6tand in the way of the current. The cold air is always flowing by the nearest road from the point at which it enters to the point at which it is consumed, that is, at the horse's nostrils. With a knowledge of this simple fact, to which I have already alluded more fully, ventilation may be so regulated that the current need not traverse much of the stable, to cool the air, nor to fall on any particular part of the horse. When the fresh air must pass over the horse, before it can reach his nostrils, its force can be broken by admitting it through numerous and wide-spread perforations, each perhaps not exceeding half an inch in diameter, but taken altogether, nearly equal in size to the aperture by which the foul air escapes.