HEAT. 



lightest, remains near the top, only sending down heat to the lower 

 part of the tube by conduction. Even long after boiling takes place 

 near the surface, no appreciable rise in temperature can be detected 



by the hand near the bottom of the tube. 

 * s ' ^ course > to ma ke use of convection 

 that boilers or vessels of water are always 

 heated from below. 



Hot- Water Heating Systems. Con- 

 vection of heat by water is used in warm- 

 ing buildings by hot-water pipes. Fig. 42 

 shows the principle on which such systems 

 are based. From the heated vessel or 

 " boiler " a pipe leads out at A, near 

 the top, and after circulating round the 

 building along B, C, D, returns into the 

 " boiler " at E, near the bottom. At 

 the highest point of the course, say 0, is 

 a cistern by which the whole system is 

 filled with water. The circulation is com- 

 FIG. 41. Boiling Water when mence d and maintained thus. The hot 

 Circulation and Convection are water rises in the boiler by convection 

 Prevented. not going sideways into the pipe at E 



and soon the pressure at E, due to the 



depth below as traced through CBA, which is partly warmed, is appre- 

 ciably less than the pressure due to the depth below C as traced through 

 ODE, which is still all cold. Hence, the cold column presses the water 

 near E into the boiler, and some of the warm water is forced into the 

 pipe AB, and the circulation is started. 

 Once started, it will evidently be kept 

 up, for the water going out of the boiler 

 is always the hottest, and therefore 

 CBAE always gives a less pressure 

 than ODE. The efficacy of the system 

 depends, to some extent, on the ver- 

 tical height of the highest point of the 

 system above E. If this is small, the 

 circulation will only be slow. It is 

 sometimes found necessary to increase 

 the height artificially by putting in a 

 vertical pipe leading from A to some 

 height above the level to be warmed, 

 and then returning down to that level. 

 Some cases of ocean currents are 

 probably examples of convection due 

 to heat, arising, however, in a slightly 

 different manner, since the heating of the ocean is chiefly from above. 

 To understand the way in which the circulation is maintained, let us 

 imagine a long canal with a horizontal bed stretching from the equator 

 at A (Fig. 43) to the pole at B. If it were all at one temperature to 

 start with, its surface would also be horizontal as AB. But through 

 equatorial heat and polar cold a rise would take place in the surface 



B 



Boiler 



S 



FIG. 42. Principle of Hot-Water 

 Heating Systems. 



