SCIENCE OF COMMON THINGS. 153 



Coolness and warmth of the air. Conduction of heat. 



better conductors of heat, and carry off the heat more 

 rapidly from the body ; but when bjr the continuance 

 of the body between them they acquire the same tem- 

 perature, they will then feel even warmer than the 

 blankets. 



989 In the summer a still, calm atmosphere feels warm, but if a wind 

 arises, the same atmosphere feels cold : has there been any real change of tem- 

 perature f 



~No ; for a thermometer suspended under shelter and 

 in a calm place w r ill indicate the same temperature as a 

 thermometer on which the wind blows. 



990 Why do we then consider that the air has grown colder ? 



Because the air in motion by the wind conducts off 

 the heat from our bodies faster than the same air at 

 rest. 



991 What is meant by the convection of heat? 



Heat communicated by being carried to another 

 thing or place ; as the hot water resting on the bottom 

 of a kettle carries heat to the water through which it 

 ascends. 



993 Are liquids good conductors of heat f 



"No ; liquids are bad conductors; and are therefore 

 made hot by convection. 



993 Why are liquids bad conductors of heat ? 



This peculiarity is referable to the mobility w r hich 

 subsists among the particles of all fluids, and to the 

 change in the size of the particles, which is invariably 

 produced by their change in temperature. 



The constituent particles of solid bodies being incapable of changing 

 their material position and arrangement, the heat can only pass through 

 them, from particle to particle, by a slow process ; but when the particles 

 forming any stratum of liquid are heated, their mass, expanding, becomes 

 lighter, bulk for bulk, than the colder stratum immediately above it, and 

 ascends, allowing the superior strata to descend. 



994 Explain hotu water is made hot. 



"When the heat enters at the bottom of a vessel con- 

 taining water, a double set of currents is immediately 

 established, one of hot particles rising towards the 



