BIVOUAC, originally meant a night-watch by a whole army under arms to prevent surprise. In modern military parlance the word is used to mean a temporary encampment in the open field without tents, as opposed to "billets" or "cantonment" on the one hand and "camp" on the other. Shelter in bivouac is usually im provised by stretching blankets or waterproof sheets on a support formed by sticks or branches driven into the ground. The use of bivouacs permits an army to remain closely concentrated for all emergencies and avoids the necessity for numerous wagons carry ing tents. Constant bivouacs, however, are trying to the health of men and horses, and this method of quartering is never employed except when the situation demands concentration and readiness.