Page:Popular Science Monthly Volume 10.djvu/688

 being deducted, the impediment by the second layer is about the same for different materials, but very considerable for each of them:

From this follows the practical truth, that we can produce a very different effect on our body by the same number of clothes, according to the tightness and looseness in the make. Just call to mind tight shoes and gloves in winter-time!

This fact leads to a series of other facts, which contain the explanation why wadding, as long as it is loose and elastic, keeps you warmer than when it is once flattened. This is the air contained within the clothes.

One generally considers clothing as an apparatus for keeping the air from us. This conception is utterly erroneous; quite the reverse, we can bear no garments which do not allow of a continual ventilation of our surface. Just those textures which are most permeable to the air keep us warmest. I have examined different materials for their permeability to air, which can be easily ascertained. One closes a series of perfectly equal glass tubes with different textures, and observes how much air passes through the clothing substances at the same pressure during the same time. Taking the quantity of air passing through flannel as 100—

to pass through them.

If our clothing kept us warm in proportion to its power of excluding the air from our body, kid would keep us a hundred times, and chamois warmer by one-half than flannel, and so on, while every one knows by experience that it is quite the reverse.

If there are several layers of the same material, ventilation loses but very little at the second layer, because the velocity of the air in its passage through the first layer remains about the same on its further progress, the following layers being like a continuation of the preceding ones, as if they were tubes of the same calibre, retarding the original velocity of a fluid by the amount only of unavoidable friction.

Thus, a current of air travels incessantly through our clothing. Its force, as in ventilation generally, depends on the size of the openings, the difference between the outside and inside temperature, and