In applying the principles of drainage to the habitations of the domestic animals there are on the whole fewer difficulties to he overcome than in the case of the human being. The effectual removal of solid and liquid excreta is the object sought in all cases; but while the sanitarian, in dealing with houses inhabited by human beings, is compelled to devise some method, not only for the removal of excreta, but also for destruction or disposal of it in such a manner that no nuisance may arise from its accumulation, the solid and liquid excreta of the horse have a commercial value as fertilizing material or manure, and are not therefore destroyed or deliberately allowed to pollute rivers and water-courses, as it pays better to store them for use. The difficulty in dealing effectually with solid and liquid excreta from the lower animals arises from the fact that the quantity voided by the larger quadrupeds is considerable, and, in respect of the solid manure, the act of excretion or expulsion is frequently performed, rendering it almost impossible to keep a large stable in a condition of even moderate cleanliness.
Emanations from animal excreta are not likely to contaminate the air of a stable or cow-shed to any serious extent while in a fresh state, but both solids and fluids rapidly undergo decomposition, the result of which is to set free certain compounds of hydrogen and other gases, which are not only offensive, but, some of them at least, poisonous - sulphuretted hydrogen, for example, arising from the solid excreta chiefly.
Urine very quickly changes its state, and sets free a quantity of ammonia in a gaseous condition. Ammoniacal gas has an intensely pungent character, and causes severe irritation of the mucous membranes of the eyes and nostrils, to an extent which can only be appreciated by those who have entered a badly ventilated or unventilated stable in which a large number of horses are habitually kept, or the holds of cattle-ships immediately after the cargo has been landed. A little experience of this kind should suffice to convince a horse-keeper of the great importance of making proper arrangements for the removal of excreta from the stable at once, no matter how often it may be necessary, to some convenient place of storage at a distance from the stable, so that the gases from the manure-heap may not be driven by winds into the stable or shed.
Stables in large towns are generally situated in rows in a long mews, and the dirty straw, with the excreta, often form a conical heap outside the stable door. Usually the sanitary authorities insist on the removal of the heaps at short intervals, so that very little opportunity is allowed for putrefactive fermentation to go on; but the same system is often adopted in the open country, where there is ample space for proper storage. The real difficulty is that the space outside the stable door is the most convenient spot for the attendants to heap the manure temporarily, or until enough has accumulated to justify the use of a cart or wagon for its removal.
In London the regulations concerning receptacles for dung are now somewhat stringent. The capacity of the receptacle must not exceed 2 cubic yards, unless "the whole of the contents . . . are removed not less frequently than every forty-eight hours"; the bottom must not be below the level of the ground; one of the sides must be readily removable to facilitate cleansing; and the receptacle must be, so constructed as to prevent rain or water from entering it, and the escape of the contents, or any soak-age therefrom, into the ground or into the wall of any building, and it must also be freely ventilated into the external air. If the dung is removed from the premises not less frequently than every forty-eight hours, a metal cage may be used as a receptacle, but the ground beneath it must be adequately paved to prevent soakage into the ground, and if the cage is placed near a building, the wall of the building must be cemented "to such an extent as will prevent any soakage from the dung . . . into the wall".
Calculations have been made of the amount of solid and liquid excreta voided by different animals in a given time, and the results have proved useful, not only in physiology but also in practical farming.
The late Professor Varnell, in the course of his observations at the Royal Veterinary College, found that a horse discharged from the body 49 lbs. of dung and 29 lbs. of urine in twenty-four hours. Col. Fred. Smith, from his own investigations, practically confirms Professor Varnell's estimate. He also records that a cow voids about 160 lbs. of dung and 18 lbs. of urine in twenty-four hours.
In different animals the consistency of the solid excreta varies very much in proportion to the amount of water it contains. Fortunately the dung of the horse in health is fairly dry, and may often be lifted from clean straw with the stable-fork or shovel, leaving hardly a trace behind it. Some horses, however - animals of an excitable temperament, - void a quantity of soft dung from time to time, and in some the habit of evacuating watery dung in small quantities at frequent intervals is maintained in spite of treatment. Such animals can hardly be considered healthy, although no other symptoms of derangement may be observed.
In the matter of the mechanics of stable drainage simplicity is all-important. The main object to be kept in view is the single one of perfect efficiency; so long as that is attained, the means employed are of secondary consequence.
Some differences in the system of drainage employed for stables in towns, as compared with those in the country, is not only permissible, but may in many cases be desirable. A town stable has, for instance, to be connected with the sewerage system, for which purpose advantage is taken of the sewer which is nearest to the stable, and the principal points to be considered are the best method of connecting the stable drain with the sewer, and whether the stable drain shall consist of an open channel or a closed one in the form of a pipe sunk a short distance underground, and running to the outside of the stable into a trap.