306. =Clothing.=--Man has no covering to guard him against cold, because he is capable of contriving clothing suitable to the various degrees of temperature to which he may be exposed. The object of clothing is not to make the body warm, but to keep it so. The heat of the body is generated continually within itself, and under all circumstances this heat is maintained quite uniformly at 98°. This, you see, is a much higher degree than the atmosphere ordinarily has. We are all the time, then, giving off heat to the air around us, except when the air gets up to 98°. We are comfortable only when we are giving off heat to a considerable amount, for the point of temperature which is most agreeable to us when we are at rest is 70° or a little less, that is nearly thirty degrees below the temperature of our bodies. When the temperature is below this we need extra clothing. In making choice of clothing for various degrees of temperature we practically apply the principles which I have developed. Those articles of clothing which can confine or entangle, as we may say, the largest quantity of air among their fibres are the best non-conductors, or, in common language, are the warmest. So, too, loose clothing is warmer than tight, on account of the amount of air between the clothing and the body. Thus a loose glove is much warmer than a tight one. The same general fact is exemplified in the coatings of straw which we put upon tender trees and shrubs in winter. It is the air that is confined in the tubes of the straw which makes these coverings so effective a defense. It is probably the air in the pores of the brick which makes it a poorer conductor than stone, as illustrated by the fact stated in § 300. 307. =Cocoons.=--Many insects pass through their pupa or transition state in cocoons. When this is done during warm weather, as in the case of the silk-worm, the cocoon is simple. But when the pupa state lasts through the winter special provisions are made in the arrangements of the cocoon to guard the insect against the cold. I will cite as an example the cocoon of one of our largest moths, the Cecropia. This cocoon, fastened to some shrub, keeps its inmate secure from the rigors of the winter by a very beautiful arrangement. The real cocoon is similar to that of the silk-worm; but it has a very dense air-tight outer covering, and the space between these two coverings of the pupa is filled with a loose substance, which has air, of course, mingled with its fibres, and acts, therefore, the part of a blanket for the insect.