Some animals, such as sheep, goats, rabbits, and hares, will drink little or no water while subsisting on green food, because they perspire only to a slight degree (Colin). The skin of horses, on the contrary, is very active, and therefore they need a comparatively large supply of water. The necessary quantity is, however, subject to wide variations, which are influenced chiefly by the nature of the forage and the activity of the skin, lungs, and kidneys. The fact that grass contains about 80 per cent. of water, and the ordinary food of stabled horses only about 25 per cent. is sufficient to show the effect of diet on the amount of water which is needed. The respective influences of work, climate, clothing, temperature of the stable, and state of the coat, are too well known to need discussion. When a horse is suffering from fever, or from having consumed too much salt or other thirst-producing substance, he will require an unusually large quantity of drinking water to aid in removing the irritating material from his body.

The daily quantity of water which a horse will drink may be estimated at from 4 to 10 gallons. In hot climates a horse which is doing no work, as on board ship, will drink 5 or 6 gallons. Under similar circumstances during cold weather, he will drink about 4 gallons.

From an experiment which Fred. Smith carried out on ten horses in India during the month of February, he found that the average was about 8 1/2 gallons, which the animals divided as follows for themselves : -

During hot weather, the average would probably have been about 10 gallons.

Horses which have to perform long and severe work, particularly when the weather is hot, might require more than twice the quantity they would usually drink.