A. The breath causes a rapid change of air to pass over the broth; and (as the air is not so hot as the broth) it keeps absorbing heat, and thus makes the broth cooler and cooler.
Q. Would not the air absorb heat from the broth just as well without blowing?
A. No; air is a very bad conductor; unless, therefore, the change be rapid, the air nearest the surface of the broth would soon become as hot as the broth itself.
Q. But would not the hot air part with its heat instantly to the circumjacent air?
A. No; not instantly. Air is so bad a conductor, that it parts with its heat very slowly: unless, therefore, the air be kept in continual motion, it would cool the broth very slowly indeed.
Q. Why does wind generally feel cool?
A. Wind is only air in motion; and the more quickly the air passes over our body, the more rapidly it absorbs the heat therefrom.
Q. Why does air absorb heat more quickly by being set in motion?
A. Because every fresh gust of air absorbs a fresh portion of heat; and the more rapid the succession of gusts, the greater will be the quantity of air absorbed.