PERIODICAL WINDS. 
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out over a distance of 10 miles at tlie equator, mov¬ 
ing poleward, is crowded into a space of only 53 
miles in width at latitude 40°. This would naturally 
heap it up in the horse latitudes, and thus, as the 
barometer shows, increase its density. 
PERIODICAL WINDS. 
Thus far I have considered the general circulation 
of the atmosphere. There are many things, however, 
which interfere with it: Just as the irregularities of 
the pebbles and boulders in the river-bed produce 
side currents and eddies in the onward flow of the 
stream, so there are various causes that disturb the 
general direction of the winds. 
I set a pan of sand and one of water in the hot 
sun. The temperature of the sand rose sooner and 
higher than that of the water, but when the sun went 
down the sand cooled faster. Why ? 
One time I camped six weeks on the shore of the 
Caribbean Sea. Scarcely a breeze could be felt at 
eight o’clock in the morning, but a half-hour later 
a gentle wind came ofl the sea, which steadily in¬ 
creased in force until about two o’clock, then it 
slowly diminished, and died out a little after sunset. 
This evening calm was soon after disturbed by the 
wind blowing seaward ofl the land, and thus contin¬ 
ued until the morning calm. 
The cause of these winds is that the land becomes 
warmer than the water during the day. The cooler 
air from the sea forces the lighter air which is over 
the land upward, thus starting a current from the sea 
