This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Another method of heating is by a similar application of direct hot water radiation. This consists in circulating hot water through radiators, which are placed in the rooms as in the case of direct steam heating. The hot water is conducted from the main heater in the cellar through the pipes and radiators, and the air is warmed by the direct radiation from these sources. The principle of this circulation is found in the difference in density and volume between hot and cold water. Water is at its greatest density at about 39 degrees F. When heated its density decreases and its volume increases, so that as soon as the fire in the boiler is started the circulation begins, and the water becoming lighter flows up through the pipes and radiators giving off its heat, until becoming colder and heavier it flows back through the return pipes to the heater. This process is continuous as long as any heat is applied to the water in the boiler, the velocity of flow depending upon the difference in temperature between the supply and return, and the height of the radiators above the boiler.