PHYSICAL FORCES 97 



extending towards us when we seek warmth by standing 

 near it. But these rays are not only given forth hori- 

 zontally ; they are radiated in all directions as may be 

 proved by suspending a red-hot sphere, when the rise in 

 temperature produced by it will be found to be equal on 

 all sides of it. The rays thus given off proceed in straight 

 lines. 



The power or intensity of heat thus radiated is like 

 the force gravity,* in that it varies inversely as the 

 square of the distance. It is four times less at two feet 

 distance than at one foot, nine times less at three feet, 



FIG. 16. 



sixteen times less at four feet, and so on. This is because 

 the heat-rays radiating from any point spread out at a 

 distance of two feet to four times the extent of space 

 they do at one foot, to nine times at three feet, sixteen 

 times at four feet, and so on. 



All bodies are constantly radiating away their heat, 

 arid as constantly receiving it from other bodies, but they 

 do this at unequal rates i.e., their radiating force is 

 unequal. This is the case even when different bodies 

 are at the same temperature. Thus mercury has only 



* See ante, p. 66, 



