"It is owing to this cause that the water at the top of a caldron, before it begins to boil, is always as warm, or warmer, than what is below:—for the particles of water that touch the bottom are no sooner acted upon by the heat below, than they become warmer, and more expanded, than those immediately above them, and therefore rise directly upwards, and give place to denser cold particles, which are forced in their turn to ascend in like manner towards the top."