QUANTITY AND TRANSMISSION OF HEAT 145 



bottom of the flask with the tip of a Bunsen burner flame. The 

 coloring matter will show the direction of the convection currents 

 (Figure 126). The water nearest the flame becomes heated and 

 expands, thus becoming less dense than the surrounding water. 

 This lighter water is then forced up by the denser water which comes 

 in from the sides to take its place. 



Convection in Gases. The winds are convection 

 currents in the atmosphere caused by the unequal heating 

 of the earth by the sun. This principle easily explains the 

 land and sea breezes near the coasts of large bodies of 

 water. During the daytime the land is heated more 

 rapidly than the water, the specific heat of water being 

 much greater than that of the earth. The hot air over 

 the land, being lighter, is forced up by the cooler air from 

 the ocean. This is the sea breeze which blows during the 

 daytime and reaches its maximum strength usually late 

 in the afternoon. At night the earth cools more rapidly 

 than the sea, and in a short time the sea is warmer than 

 the land and the current of air is reversed. This is the 

 land breeze which blows during the night and reaches its 

 maximum toward morning. These winds are more notice- 

 able in the tropics, since the change in temperature from 

 day to night is greatest there. 



Radiation. When we stand before a fireplace, it is 

 evident that we are receiving heat that comes to us 

 neither by conduction nor convection. It cannot be due 

 to conduction, because the conductivity of air is very 

 small. It cannot be due to convection, because the cur- 

 rents of air are moving toward the fire instead of away 

 from it. There must therefore be some way in which heat 

 travels across space other than by conduction or con- 

 vection. This third method of heat transference in which 

 the heat emanates in straight lines from a source inde- 



