157. Why would you apply the heat at the top, in this experiment?
Because in heating water it expands and rises. The boiling of water is caused by the heated water ascending from the bottom, and the colder water descending to occupy its place. If the heat were not applied at the top, it would be distributed quickly by convection, but not by conduction.
158. Why are bottles of hot water, used as feet-warmers, wrapped in flannel?
Because the flannel, being a bad conductor, allows the heat to pass only gently from the bottle, and preserves the warmth for a much longer time.
159. Why are hot rolls sent out by the bakers, wrapped up in flannel?
Because the flannel, being a bad conductor, does not carry off rapidly the heat of the rolls.
160. Why is it said that snow keeps the earth warm?
Because snow is a bad conductor, and prevents the frosty air from depriving the earth of its warmth.
161. Why are snow huts which the Esquimaux build found to be warm?