the interstices between its fibres cease to exist; and the air which they contained and which is a powerful non-conductor of heat, is squeezed out.  Consequently wherever the blanket is compressed, its power of retaining the heat of the sleeper is diminished.  Soft fleecy substances, like eider-down quilts, which are extremely warm as coverlets, are well-nigh useless as mattresses.  There is another cause why a sleeper requires more protection from below, than from above:  it is that if the ground be at all wet, its damp will penetrate through very thick substances laid upon it.  It will therefore be clearly understood that the object of a mattress is not alone to give softness to the bed, but also to give warmth; and that if a man lies in a hammock, with only the hammock below, and blankets above, he will be fully as much chilled as if the arrangement had been reversed, and he had lain upon blankets, with only the hammock as a sheet to cover him.

Vital Heat.—­The vital heat of a man, either in an active or a latent form, is equal to that which is given out by two ordinary candles:  I judge so from the following reasons.  All our vital heat is produced by the combustion—­for it is simple combustion—­of the carbon in our food.  Now the quantity of carbon consumed by a man in full diet, in 24 hours, is about 22 oz. in weight.  On the other hand, I find that ordinary candles, which mainly consist of carbon, burn at the rate of 11 oz. in 24 hours.  Therefore the heat given out by two candles is just about the same as that given out by one man, either in a sensible form, or else under a latent form by the vapour of the breath.  Secondly, I have frequently heard it estimated, as the result of the ordinary experience of social life, that a saloon is warmed by each couple of candles somewhat more than it is by the presence of a single guest.  Where I write these lines, I have not an opportunity of verifying my rough estimate, by reference to physiological works, but accuracy is of little consequence to my present purpose, which is to give a general idea of the magnitude of the problem to be solved by clothes and tenting.  Their joint office is to retain the heat of a mass of flesh and blood, the size and shape of a man, warmed by two candles burning within it, at a temperature of not less than 96 degrees in its inward parts.

Mattresses and their Substitutes.—­A Strip of Macintosh.—­If a traveller can do so, he should make a point of having a strip of macintosh sheeting 7 feet by 4, certainly not less than 6 feet by 3, to lay on the ground below his bedding.  Every white servant in the expedition ought to be furnished with a strip of macintosh sheeting, or, failing that, with a strip of painted canvas.  However, painted cloth is much inferior to macintosh, as it will not fold up without cracking:  it also tears easily, and is heavy.  Macintosh, of the sort that suits all climates, and made of linen, not of silk, is invaluable to an explorer, whether in the form of sheeting, coats, water-bags. swimming belts, or inflatable boats.  A little box full of the composition for mending it, and a spare bit of macintosh, should always be taken.