Page:Elementary Text-book of Physics (Anthony, 1897).djvu/215

 CHAPTER II.

TRANSFER OF HEAT.

172. Transfer of Heat.—In the preceding discussions it has been assumed that heat may be transferred from one body to another, and that if two bodies in contact be at different temperatures, heat will be transferred from the hotter to the colder body. In general, if transfer of heat be possible in any system, heat will pass from the hotter to the colder parts of the system, and the temperature of the system will tend to become uniform. There are three ways in which this transfer is accomplished, called respectively convection, conduction, and radiation.

173. Convection.—If a vessel containing any fluid be heated at the bottom, the bottom layers become less dense than those above, producing a condition of instability. The lighter portions of the fluid rise, and the heavier portions from above, coming to the bottom, are in their turn heated. Hence continuous currents are caused. This process is called convection. By this process, masses of fluid, although fluids are poor conductors, may be rapidly heated. Water is often heated in a reservoir at a distance from the source of heat by the circulation produced in pipes leading to the source of heat and back. The winds and the great currents of the ocean are convection currents. An interesting result follows from the fact that water has a maximum density (§ 190). When the water of lakes cools in winter, currents are set up and maintained, so long as the surface water becomes more dense by cooling, or until the whole mass reaches 4°. Any further cooling makes the surface water lighter. It therefore remains at the surface, and its temperature