in the Air and in the Sea. 25 



parallels, therefore entirely as with the compensation-currents, 

 constant west winds prevail, under the name of anti-trades. 

 Between these constant air-currents there are, just as with the 

 sea-currents, both in the vicinity of the equator and not far from 

 the 30th degree of latitude in both hemispheres, zones of no 

 wind, which are known by the name of the equatorial and the 

 tropical calms. The trade-winds, with the calm-zones belonging 

 to them, shift a little with the seasons — in the summer of the 

 northern hemisphere moving somewhat northward, and in the 

 winter toward the south. In the currents of the ocean this 

 shifting of the zones is less marked; hence the currents of 

 water and air do not exactly correspond; yet, excepting slight 

 deviations, the trade-winds with their calms present the same 

 picture as the equatorial currents with their streamless zones. 

 In spite, however, of this striking similarity, the trade-winds 

 have till now been ascribed to quite different causes from those 

 of the great rotation-currents of the ocean. 



We shall presently return to this subject; we will now only 

 mention further that the rest of the great currents in the dif- 

 ferent oceans likewise correspond so completely that the exist- 

 ence of very determinate laws must thence be inferred. Thus 

 the Gulf-stream and the Japanese Kurosiwo exhibit precisely 

 the same phenomena : both are currents of warm water, flow in 

 a north-easterly direction, and are separated from the coast to 

 the west of their course by a cold current flowing in the oppo- 

 site direction. The cold current of Peru corresponds to that of 

 South Guinea, just as does the warm Brazilian current to that 

 of Mozambique. Lastly, a feeble current from south-west to 

 north-east prevails in the entire south polar sea. 



The trade-winds have from the 17th century been represented 

 as polar winds which are deflected from the direction of the meri- 

 dian by the earth's rotation : this theory was, as far as we know, 

 first set up (very imperfectly, it is true) by Varenius, in 1650; 

 it was subsequently improved by Halley in 1686, and Hadley 

 in 1735, and is for the most part named after the latter, as it 

 has since made no advance. That this theory has been so gene- 

 rally accepted is so much the more surprising, as many pheno- 

 mena of the trade-winds are scarcely in accordance with it. 



According to this theory, the masses of air in the equatorial 

 regions, rendered lighter by heating, are continually ascending, 

 through which the cooler and heavier air of higher latitudes is 

 impelled to flow toward the equator. As the velocity of rota- 

 tion is greater at the equator than at any other latitude, and 

 gradually diminishes to the poles, while the air-particles (by the 

 law of inertia) do not at once take up this greater velocity as 

 soon as they arrive at parallel circles where the motion is more 



