"When heat is passing through a body by conduction, the temperature of the body must be greater in the parts from which the heat comes than in those to which it tends, and the quantity of heat which passes through any thin layer of the substance depends on the difference of the temperatures of the opposite sides of the layer. ...[W]e may define conduction as the passage of heat through a body depending on inequality of temperature in adjacent parts of the body. ...[T]he parts of the body through which the heat comes... must be hotter... and the parts higher up the stream of heat still hotter."