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124   Art of Travel.

return to civilisation, instead of complacently interesting himself in its continuance. This is a frame of mind in which few great journeys have been successfully accomplished ; and an explorer who cannot divest himself of it, may suspect that he has mistaken his vocation.

It is a common idea among men who are preparing to travel for the first time, that all the bed-clothing about which they need concern themselves, is a sufficiency to cover them, forgetting that a man has an under as well as an upper side to keep warm, and must therefore have clothing between him and the earth, as well as between him and the air. Indeed, on trying the experiment, and rolling oneself up in a single blanket, the undermost side in a cold night is found to be by far the colder of the two. The substance of the blanket is compressed by the weight of the sleeper ; 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

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