122 TRADE WINDS. SECT. XV. 



seldom less than an inch, sometimes double that quantity. The 

 great crest is preceded and followed at about five days' interval 

 by two lower ones, and the beginning and end are marked by 

 deep depressions. The researches of M. Leverrier leave no doubt 

 that the great Crimean storm of the 14th November, 1854, was 

 part of this phenomenon,* for even a very small difference of 

 atmospheric pressure is sufficient to raise a considerable wind. 

 Since each oscillation has its perfect effect independently of the 

 others, each one is marked by a change in the barometer, and 

 this is beautifully illustrated by curves constructed from a series 

 of observations. The general form of the curve shows the course 

 of the principal wave, while small undulations in its outline 

 mark the maxima and minima of the minor oscillations. 



The trade-winds, which are the principal currents in the 

 atmosphere, are only a particular case of those very general 

 laws which regulate the motion of the winds depending on the 

 rarefaction of the air combined with the rotation of the earth on 

 its axis. They are permanent currents of wind between the 

 tropics, blowing to the N.E. on the N. side of the equator, 

 and to the S.E. on the S. side. 



If currents of air come from the poles, it is clear that equili- 

 brium must be restored by counter-currents from the equator ; 

 moreover, winds coming from the poles, where there is no rota- 

 tion, to the equator, which revolves from W. to E. at the rate 

 of 1000 miles an hour, must of necessity move in a direction 

 resulting from their own progressive motion and that of rota- 

 tion ; hence, in blowing towards the equator the bias is to the 

 E., and in blowing from it the bias is to the W. Thus as N. 

 and S. winds from the poles blow along the surface from the 

 tropics to the equator, in consequence of this composition of 

 motions that from the N. becomes the N.E. trade-wind, and 

 that from the S. the S.E. trade-wind. Now these winds being 

 in contrary directions cross at the equator, balance each other 

 through about 6 degrees of latitude, and produce a belt of calms 

 of that breadth encircling the globe, known as the calms of the 

 equator, or the Variables of seamen. The heat of the sun 

 rarefies the air so much, that the trade-winds, after crossing at 

 the equator, ascend into the higher regions of the atmosphere, 

 where that from the N. goes to the tropic of Capricorn, and 

 * Sir John Herschel on Meteorology. 



