The Propagation of Heat 193 



air itself in this region is not heated by the passage 

 of the wave-motion may be shown by interposing a 

 screen, such as a sheet of paper, between the hand 

 and the hot body. An immediate feeling of coolness 

 is experienced, which could hardly be the case if the 

 air itself were hot. 



EXPERIMENT 71. Suspend the metal ball used in 

 Experiment 40 in the middle of the flame produced 

 by a powerful Bunsen burner. While the ball is being 

 heated, hang two or three thermometers from the rings 

 of retort stands so that the bulbs of the instruments 

 are at the same level as the centre of the ball, and at 

 different distances from the flame, say 10", 15", and 20". 

 Each thermometer will record an increase of tempera- 

 ture, that nearest the flame showing the greatest 

 change, and the most distant one the least. When the 

 thermometer readings no longer increase turn out the 

 burner and quickly remove the hot ball. Each will 

 almost immediately register loss of heat as shown by 

 the fall of mercury, denoting that the air in the neigh- 

 bourhood of the instruments has not been made hot. 

 This experiment may be varied by placing three ther- 

 mometers at equal distances from the hot ball, after 

 removal of the flame, one being placed 10" above the 

 ball, a second 10" below it, and a third 10" away in 

 the same horizontal plane. 



Exercise for Student. 



Account for any differences in the readings noted in the experi- 

 ment just suggested. 



An everyday application of the observations made 

 in our experiment is to be found in the action of a 

 person seated near a hot fire and seeking relief from 

 H. D. s. 13 



