[Verse: "He giveth snow like wool: he scattereth the hoar frost like ashes."--PSALM CXLVII.]
[Illustration: CRYSTALS OF SNOW, AS SEEN THROUGH A MICROSCOPE. FIG. 1.]
163. _Why does it frequently feel warmer after a frost has set in?_
Because, in the act of congealation a great deal of heat is given out, and _taken up by the air_, and thus _the severity of the cold is in some degree moderated_.
164. _Why is it frequently colder when a thaw takes place?_
Because, in the process of thawing, a certain amount of heat is _withdrawn from the air_, and enters the thawed ice.
165. _What benefit results from these provisions of Nature?_
They moderate both the _severity of frosts_, and _the rapidity of thaws_, which, in changeable climates, would be seriously detrimental to _life_, and to _vegetation_.
166. _Why are furs and woollens worn in the winter?_
Because, being non-conductors, they prevent the warmth of the body from being _taken up by the cold air_.
167. _Why are the skins of animals usually covered with fur, hair, wool, or feathers?_
Because their coverings, being _non-conductors of heat_, preserve the warmth of the bodies of the animals.
[Verse: "He sendeth out his word, and melteth them: he causeth his wind to blow, and the waters to flow."--PSALM CXLVII.]
168. _How is the greater warmth of animals provided for in the winter?_
It is observed that, as winter approaches, there comes a short woolly or downy growth, which, _adding to the non-conducting property of their coats_, confines their animal warmth.
In small birds during winter, let the external colour of the feathers be what it may, there will be found a kind of _black_ down next their bodies. Black is the _warmest colour_, and the purpose here is to _keep in the heat_, arising from the respiration of the animal.
169. _How is warmth provided for in animals that have no such coats?_
They are furnished with a layer of _fat_, which lies underneath the skin. Fat consists chiefly of _carbon_, and is a _non-conductor_.
170. _Why are summer breezes said to be cool?_
Because, as they pass over the heated surface of the body, they bear away a part of its heat.
171. _Why is a still summer air said to be sultry?_
Because, being heated by the sun's rays, _and being a bad conductor_, it does not relieve the body by _carrying off its heat_.