Recognized HTML document

Bivouac.

133

gladly, notwithstanding the apparent insignificance of its cause.

Shelter from the Sky.-The shelter of a wall is only sufficient against wind or driving rain ; we require a roof to shield us against vertical rain, and against dew, or what is much the same thing, against the cold of a clear blue sky on a still night. The temperature of the heavens is known pretty accurately, by more than one method of calculation : it is - 289° Fahr. ; the greatest cold felt in the Arctic regions being about - 40° Fahr. If the night be cloudy, each cloud is a roof to keep off the cold; if it be clear, we are exposed to the full chill of the blue sky, with only such.alleviation as the warming and the non-conducting powers of the atmosphere may afford. The effect is greater than most people would credit. The uppermost layer of the earth, or whatever may be lying exposed upon it, is called upon to part with a great quantity of heat. If it so happen that the uppermost layer is of a non-conducting nature, the heat abstracted from it will be poorly resupplied by communication from the lower ones. Again, if the night be a very calm one, there will be no supply of warmth from fresh currents of air falling down upon it. Hence, in the treble event of a clear blue sky, a non-conducting soil, and a perfectly still night, we are liable to have great cold on the surface of the ground. This is shared by a thin layer of air that immediately rests upon it ; while at each successive inch in height, the air becomes more nearly of its proper temperature. A vast number of experiments have been made by Mr. Glaisher on this subject (` Phil. Trans.' 1847), the upshot of which is that a thermometer laid on grass, under a blue sky on a calm night, marks on an average 8° Fahr. colder than one 4 feet above it ; 1 inch above grass, 52°; 1 foot, 1°; 4 feet, 2°; on gravel and sand'the differences are only about one-third as much. Sheep have a practical knowledge of these differences. Often, in an early walk on dewy mornings, I see all the sheep in Hyde Park bivouacked on the gravel walks or Rotten Row. The above figures are the results of experiments made in England, where the air is always moist, and the formation of dew, while it testifies to the cold of the night, assists largely to moderate it. In arid