438 Transactiom. — Geology. 



and becoming the north-cast trade wind, blowing towards the south-west until 

 it also reaches the equatorial belt of calms. This belt of calms is caused by the 

 contact of these two currents. At this point both begin to ascend, and, 

 becoming cooler by expansion in the upper region, a considerable projiortion of 

 the moisture which they have absorbed as trade winds is precipitated. 

 According to Maury, each of these upward currents, after passing over the 

 tropical region between the equatorial and solstitial belts of calms, again 

 plunges at the latter, passing under the descending cold currents from the polar 

 regions. Whilst thus plunging a further proportion of the moisture left in 

 them is condensed, causing that peculiar cloudy character which distinguishes 

 the districts between the limits of the solstitial calm belts. After this plunge 

 they again become surface currents, flowing towards the poles in spiral or 

 loxodromic curves, in order to replace the material of the ujDper polar currents 

 above referred to, but in directly oj^posite directions, the north-east trade wind 

 becoming a north-west wind to the south of the thirty-third parallel of south 

 latitude, and the south-east trade becoming a south-west wind to the northward 

 of the thirtieth parallel of north latitude. Those two return currents are then 

 called the anti-trades. 



Sir John Herschell, who differs in some respects from Maury, says : — 

 " The trade winds occupy two belts on the earth's surface, on either side of the 

 equator, which are limited on the equatorial side by a belt of calm air, the 

 movement of which is upwards, and in which no prevailing tendency east or 

 west is perceivable. On the polar they are limited by two belts of compara- 

 tive calm, with uncertain and variable winds, which for our present pui'i:>ose 

 we may consider as nearly coincident with the tropics. Over the belt of 

 equatorial calms, the north-east and south-east trades, reduced to meridional 

 directions hy the eastward frictional impulse of the earth's rotation (Basil 

 Hall, Fragments of Voyages and Travels, 2nd series, 162) meet, and, to a 

 certain small extent perhaps, commingle in their upward movement, which, 

 however, can only be the case with those portions of air which actually attain 

 the medial line or approach very near it ; for, as the regions of calm extend to 

 four or jBve degrees on either side of that line, the greater part by far of either 

 indraught will rise on its own side, and must of necessity be turned over 

 towards the pole of its own denomination, and return as an upper current by 

 a track precisely the reverse of that of its arrival. On the other hand, over 

 the region of tropical calms a poition of the descending air of the upper 

 current, where it first strikes the earth, is dragged back into the tropical 

 circulation, wliile the rest goes forward to form the anti-trades (or south-west 

 and north-west winds) of the temperate and polar zones, which, as prevalent 

 winds, with more or less frequent interrui)tions according to local circumstances, 

 occupy both the extra-tropical regions." 



