(2) The thorax and organs cramped and lifted by pressure of the clothing.

(From an X-ray photograph .-- After Dickinson.)] No clothing, of course,--not even fur,--has any warmth in itself; it simply has the power of retaining, or keeping in, the warmth of the body that it covers.

The best and most effective way of retaining the body warmth is to surround the body with a layer of dead, or still, air, which is the best non-conductor of heat known.

Hence, porous garments, like loosely-woven flannels, blankets, and other woolen cloths, are warm because they contain or hold large amounts of air in their spongy mesh.
The reason why furs are so warm is that their soft, furry under-hairs, or "pelt" as the furriers call it, entangle and hold an enormous amount of air.

The fur of ordinary sealskin, for instance, is about half an inch deep; and _ninety per cent_ of this half-inch is air.