In the severe climate of Thibet, Dr. Hooker informs us that they encamp near large rocks, which absorb the heat during the day, and give it out slowly during the night. They form, as it were, reservoirs of caloric, the influence of which is exceedingly grateful during a cold night. In the polar regions the Esquimaux live and make themselves comfortable in huts of ice or snow, and with no other combustible but oil. The natives of Australia bury their bodies in the sand, keeping their heads only above the surface, and thus sleep warm during the chilly nights of that climate. Fortunately for the health and comfort of travelers upon the Plains, the atmosphere is pure and dry during the greater part of the year, and it is seldom that any rain or dew is seen; neither are there marshes or ponds of stagnant water to generate putrid exhalations and poisonous malaria. The night air of the summer months is soft, exhilarating, and delightful. Persons may therefore sleep in it and inhale it with perfect impunity, and, indeed, many prefer this to breathing the confined atmosphere of a house or tent. During the rainy season only is it necessary to seek shelter. In traveling with covered wagons one always has protection from storms, but with pack trains it becomes necessary to improvise the best substitutes for tents. A very secure protection against storms may be constructed by planting firmly in the ground two upright poles, with forks at their tops, and crossing them with a light pole laid in the forks. A gutta-percha cloth, or sheet of canvas, or, in the absence of either of these two, blankets, may be attached by one side to the horizontal pole, the opposite edge being stretched out to the windward at an angle of about forty-five degrees to the ground, and there fastened with wooden pins, or with buckskin strings tied to the lower border of the cloth and to pegs driven firmly into the earth. This forms a shelter for three or four men, and is a good defense against winds and rains. If a fire be then made in front, the smoke will be carried away, so as not to incommode the occupants of the bivouac. This is called a "half-faced" camp. [Illustration: HALF-FACED CAMP.] Another method practiced a great deal among mountain men and Indians consists in placing several rough poles equidistant around in a half circle, and bringing the small ends together at the top, where they are bound with a thong. This forms the conical frame-work of the bivouac, which, when covered with a cloth stretched around it, makes a very good shelter, and is preferable to the half-faced camp, because the sides are covered. [Illustration: CONICAL BIVOUAC.]