vin Atmospheric Phenomena 125 



12 N., while the south-east trades do not extend so far 

 south, but cross the equator, blowing as far as 5 or even 8 

 N. The calm equatorial zone of rains always lies north of 

 the equator, on account of the heating influence of the greater 

 mass of land in the northern hemisphere. (Plate VII.) 



1 80. The Roaring Forties is a name given by sailors 

 to the belt of ocean between 40 and 50 S. in which the 

 " Brave West Winds " blow all the year round, as regularly 

 as the trades and more strongly. This belt is more nearly 

 covered with a uniform stretch of ocean than any other 

 part of the Earth, and exhibits the theoretical circulation of 

 the atmosphere in great perfection. The prevailing wind is 

 produced by the high pressure of the south tropical calm 

 'belt and the remarkably low pressure which surrounds the 

 south pole. The strength and constancy of the brave west 

 winds enable sailing vessels to compete with steamers in 

 trading with New Zealand going by the Cape of Good Hope 

 and returning by Cape Horn. 



1 8 1. The Northern Anti-trades. The south-west 

 winds of the northern hemisphere, which blow from the 

 northward edge of the north tropical zone of high pressure 

 to the north polar region of low pressure, are sometimes 

 called the Anti-trades ; but they are much less constant and 

 more variable in strength than the trade winds or the winds 

 of the Roaring Forties. The trade winds blowing into the 

 Gulf of Mexico in the summer months from the east or 

 south-east are deflected by the edge of the great table- 

 lands of Mexico into south-westerly winds, which blow up the 

 Mississippi valley and sweep across the Atlantic, reinforcing 

 the somewhat uncertain anti-trades. 



182. Daily Temperature Changes. The circulation of 

 the atmosphere which has just been described was deduced 

 by mathematical reasoning from a few simple data, and then 

 proved by observation to be correct so far as disturbing 

 causes allow. But the changes in the atmosphere which 

 take place from hour to hour throughout the day were first 

 observed in thousands of cases, and their cause has been 

 subsequently ascertained by inductive reasoning ( 17). 

 Solar radiation goes on from sunrise to sunset, but the 



