elf 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 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