348 MODES OF DIFFUSING SEAT. 



perature. There is no chance for convection ; conduction acts much 

 more slowly. 



547. Rectilinear Propagation. Radiant heat 

 travels in straight lines through any uniform 

 medium. 



(a.) Between any source of heat and a thermometer place several 

 screens. If holes be made in the screens (See Fig. 272) so that a 

 straight line from the source of heat to the thermometer may pass 

 through them, the thermometer will be affected by the heat. By 

 moving one of the screens so that its opening is at one side of this 

 line, the heat is excluded. In a very warm day a person may step 

 from a sunny into a shady place for the same reason. The heat that 

 moves along a single line is called a ray of heat. 



548. Radiation Equal in all Directions. 



Heat is radiated equally in all directions. If an 

 iron sphere or a kettle of water be heated, and delicate 

 thermometers placed on different sides of it at equal dis- 

 tances, they will all indicate the same temperature. 



549. Radiation Depends upon Tempera- 

 ture of the Source. Tlie intensity of radiant 

 heat is proportional to the temperature of the 

 source. 



(a.) Near a differential thermometer, place a vessel of water 10 

 warmer than the temperature of the room. Notice the eifect upon 

 the thermometer. Heat the water 10 more and repeat the experi- 

 ment at the same distance. Then heat the water 10 still more and 

 repeat the experiment again. The effects upon the thermometer will 

 be as the numbers one, two and three. 



550. Effect of Distance. TJie intensity of 

 radiant heat varies inversely as the square of the 

 distance. 



(a.) Place the differential thermometer at a certain distance from 

 the heated water and note the effect. Removing the thermometer 

 to twice that distance the effect is only one-fourth as great, etc. 



