132 SCIENCE OF COMMON THINGS 



Sources of heat. The sun a source of heat. Burning-glasses. 



while at the poles it is believed to be about 13 below 

 zero. 



" Average temperature" that is, the mean or medium temperature. 

 "ZERO" the point from which a thermometer is graduated; it is 32' 

 below freezing, Fahrenheit's thermometer. 



83 O How many sources of heat are recognised to exist t 



Six. 



831 What are they f 



1. The sun: 2. The interior of the earth 3. Electri- 

 city ; 4^ Vital action ; 5. Mechanical action 6. Che- 

 mical action. 



CHAPTEE II 



THE SUN A SOURCE OF HEAT- 

 83S What is the great natural source of heat f 



The sun. 



833 Why do burning-glasses set fire to substances submitted to their 

 power? 



Because, when the rays of the sun 

 pass through the burning-glass, they 

 are bent towards one point, called 

 the^ " focus ;" in consequence of 

 _ . _ which the light and heat at this 

 ~Fi*35. point are very greatly increased. 



Fig. 35 represents the manner in which a burning-glass concentrates 

 or bends down the rays of heat until they meet in a focus. 



834 Do the rays of the sun ever set fire to natural substances without 

 the assistance of a burning-glass f 



No ; the rays of the sun, even in the torrid zone, are 

 never hot enough to kindle natural substances, unless 

 concentrated by a "burning-glass. 



835 Does the heat of the sun possess any different properties from arti- 

 ficial heat? 



