WINDS. 
109 
ground. Other air near and above it slipped down 
and followed after it, producing a circulation as shown 
in Fig. 57. 
I observed four things about the air in connec¬ 
tion with the bonfire: there was an upward move¬ 
ment of the air just above 
the fire; an inward move¬ 
ment along the ground to¬ 
ward the fire (Fig. 57); an 
outward movement, some 
distance above the fire, of 
the air toward either side; 
and a settling of the air, 
which had now become 
cold, at some distance from 
the fire (Fig. 57). The bon¬ 
fire suggested to me the 
philosophy of the winds: 
the air is heated in some 
places more than in others, 
and a current sets in from the colder toward the 
warmer areas. If the temperature were the same in 
all parts of the atmosphere there would be eternal calm. 
The sun, which shines more or less vertically in all 
regions of the equator, heats the earth there more 
than anywhere else, and the earth in turn heats the 
atmosphere. Therefore the colder air immediately 
to the north and to the south of the equator rushes in 
and pushes up the warmed air. As the colder air 
moves toward the equator it leaves a partial vacuum, 
into which the air back of it flows. 
Fig. 58.—Air moving toward 
equatorial region and re¬ 
turning to the poles as up¬ 
per air currents. 
