98 EXPERIMENTAL GENERAL SCIENCE 



to be heated. As it gives off its heat its density becomes 

 greater and it finally runs back to the boiler where it is reheated 

 and goes on its rounds again. Probably the greatest convection 

 circuits of which we have any knowledge are such ocean cur- 

 rents as the Gulf Stream and Japan Current which, heated 

 by the sun near the equator, flow away above the colder, heav- 

 ier water of the ocean and, after giving up their heat to more 

 northern regions, settle down and slowly return to the equator. 

 88. Convection and Frost. When the air cools at night, it 

 begins to settle down into the hollows, pushing out the warmer 

 and lighter air which flows upward along the ground. For 

 this reason the hillsides often escape frost which injures plants 

 in the valley. When the conformation of a country is suitable, 

 the cold air may flow out of a valley like an invisible river. 

 Such regions are especially adapted to fruit growing since they 

 are not subject to such great danger from frost. When the 

 wind blows, the moving air prevents the cooler air from settling 

 in one place and so protects from frost. 



Practical Exercises 



1. Cut two holes about an inch square and several inches apart in 

 the cover of a shallow box, such as a cigar box, and over each hole set 

 a lamp chimney. Below one of the openings place a lighted candle end. 

 Light a piece of touch paper, or anything else that will make consider- 

 able smoke, and hold it first over one chimney and then over the other. 

 Account for the direction of the air current in each chimney. 



2. Why are refrigerators usually built with the space for ice at the 

 top? 



3. Which of our systems of heating makes use of convection currents 

 in the air? 



4. Would a hot-air furnace warm the house as well if placed in the 

 attic instead of in the basement? Why? 



