Q. Why is the tin foot-warmer covered with flannel?
A. 1st—To prevent the perspiration of the foot from taking off the polish of the tin:
2ndly—Flannel is a very bad conductor; and, therefore, helps to keep the tin hot longer: and
3rdly—If the feet were not protected, the conducting surface of the tin would feel painfully hot.
Q. What harm would it be if the polish of the tin were injured by the perspiration of our feet?
A. Polished tin throws off its heat very slowly; but dull, scratched, painted, or dirty tin, throws off its heat very quickly: if, therefore, the tin foot-warmer were to lose its polish, it would get cold in a much shorter time.
Q. Why are furnaces and stoves (where much heat is required) built of porous brick?
A. As bricks are bad conductors, they prevent the escape of heat: and are, therefore, employed where great heat is required.
Q. Why are furnace doors, &c. frequently covered with a paste of clay and sand?