BIVOUAC.
There are four ways in which travellers who are thrown upon their own resources may house themselves. They may bivouac, that is to say, they may erect a temporary shelter of a makeshift character, partly from materials found on the spot, and partly from the cloths they may happen to possess; they may build a substantial hut, which of course takes a good deal of labour to complete; they may use sleeping-bags; or they may pitch a regular tent. I will speak of these four methods of encamping, --the bivouac, the hut, the sleeping-bag, and the tent, in that order.
General Remarks.--Bivouacking is miserable work in a wet or unhealthy climate; but in a dry and healthy one, there is no question of its superiority over tenting. Men who sleep habitually in the open, breathe fresher air and are far more imbued with the spirit of wild life, than those who pass the night within the stuffy enclosure of a tent. It is an endless pleasure to lie half awake watching the stars above, and the picturesque groupings of the encampment round about, and to hear on all sides the stirrings of animal life. And later in the night, when the fire is low, and servants and cattle are asleep, and there is no sound but of the wind and an occasional plaintive cry of wild animals, the traveller finds himself in that close communion with nature which is the true charm of wild travel. Now all this pleasure is lost by sleeping in a tent. Tent life is semi-civilization, and perpetuates its habits. This may be illustrated by a simple trait; a man who has lived much in bivouacs, if there be a night alarm, runs naturally into the dark for safety, just as a wild animal would; but a man who travels with tents becomes frightened when away from its lights, or from the fancied security of its walls.
In a dangerous country there can be no comparison between the hazard of a tent and that of a bivouac. In the former a man's sleep is heavy; he cannot hear nearly so well; he can see nothing; his cattle may all decamp; while marauders know exactly where he is lying, and may make their plans accordingly. They may creep up unobserved and spear him through the canvas. The first Napoleon had a great opinion of the advantages of bivouacking over those of tenting. He said it was the healthier of the two for soldiers. (See p. 153.)
Shelter from the Wind.--Study the form of a hare! In the flattest and most unpromising of fields, the creature will have availed herself of some little hollow to the lee of an insignificant tuft of grass, and there she will have nestled and fidgeted about till she has made a smooth, round, grassy bed, compact and fitted to her shape, where she may curl herself snugly up, and cower down below the level of the cutting night wind. Follow her example. A man, as he lies upon his mother earth, is an object so small and low that a screen of eighteen inches high will guard him securely from the strength of a storm. A common mistake of a novice lies in selecting a tree for his camping-place, which spreads out nobly above, but affords no other shelter from the wind than that of its bare stem below.
It may be, that as he walks about in search of shelter, a mass of foliage at the level of his eye, with its broad shadow, attracts him, and as he stands to the leeward of it it seems snug, and, therefore, without further reflection, he orders his bed to be spread at the foot of some tree. But as soon as he lies down on the ground the tree proves worthless as a screen against the wind; it is a roof, but it is not a wall. The real want in blowy weather is a dense low screen, perfectly wind-tight, as high as the knee above the ground. Thus, if a traveller has to encamp on a bare turf plain, he need only turn up a sod seven feet long by two feet wide, and if he succeeds in propping it on its edge, it will form a sufficient shield against the wind.
In heavy gales, the neighbourhood of a solitary tree is a positive nuisance. It creates a violent eddy of wind, that leaves palpable evidence of its existence. Thus, in corn-fields, it is a common result of a storm to batter the corn quite flat in circles round each tree that stands in the field, while elsewhere no injury takes place. This very morning that I am writing these remarks, November 158, I was forcibly struck by the appearance of Kensington Gardens, after last night's gale, which had covered the ground with an extraordinary amount of dead leaves. They lay in a remarkably uniform layer, of from three to five inches in depth, except that round each and every tree the ground was absolutely bare of leaves for a radius of about a yard. The effect was as though circular discs had been cut out, leaving the edges of the layer of leaves perfectly sharp and vertical. It would have been a dangerous mistake to have slept that night at the foot of any one of those trees.
Again, in selecting a place for bivouac, we must bear in mind that a gale never blows in level currents, but in all kinds of curls and eddies, as the driving of a dust-storm, or the vagaries of bits of straw caught up by the wind, unmistakably show us. Little hillocks or undulations, combined with the general lay of the ground, are a chief cause of these eddies; they entirely divert the current of the wind from particular spots. Such spots should be looked for; they are discovered by watching the grass or the sand that lies on the ground. If the surface be quiet in one place, while all around it is agitated by the wind, we shall not be far wrong in selecting that place for our bed, however unprotected it may seem in other respects. It is constantly remarked, that a very slight mound or ridge will shelter the ground for many feet behind it; and an old campaigner will accept such shelter gladly, notwithstanding the apparent insignificances 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 -239 degrees Fahr.; the greatest cold felt in the Arctic regions being about -40 degrees 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 degrees Fahr. colder than one 4 feet above it; 1 inch above grass, 5 1/2 degrees; 1 foot, 1 degrees; 4 feet, 1/2 degrees; 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 of 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 climates the chill would be far greater; such would also be the case at high elevations. One of Mr. Glaisher's experiments showed a difference of no less than 28 degrees between the cold on the ground and that at 8 feet high. This might often be rivalled in an elevated desert, as in that of Mongolia. Hence the value of the protection of a roof and of a raised sleeping-place, to a man sleeping under a blue sky in still weather, admits of easy interpretation.