140 WINDS OF THE NORTHERN IIE5IISPIIERE. 



earth, we see that it must be quite impossible for the upper <and lower currents to 

 How iu opposite directions, one upon the other, for so great a distance, without inter- 

 mingling. Each must communicate to the other its own motion by friction, and it 

 will be only near the northern and southern extremities of the vortex, that they 

 will be kept distinct, and each have its own proper motion. In other words, there 

 must be a system of equatorial winds at the south, a system of polar ones at the 

 north, and a system of blended ones between. Let us notice the necessary cha- 

 racteristics of each system separately. 



The lower current of the equatorial system proceeding southerly, and at the 

 same time veering toward the west, constitutes the trade-winds, and it is probable 

 that at the limit where the upper current becomes blended with the lower, no 

 inconsiderable part of it folds under itself, and returns toward the equator, thus 

 contributing to augment the strength of the "trades." This limit we suppose to 

 be that which defines the northern boundary of the equatorial winds on Plates I. 

 and VII. 



On the same principles, the cold surface wind of the arctic regions must com- 

 mence to flow southerly — must veer toward the west like the trade-winds, and for 

 the same reason — and finally become blended with the winds of the intermediate 

 system; thus defining the southern limits of the polar winds. 



In this intermediate system of blended winds, the mean direction must be the 

 result of two opposite motions, the upper current tending to move eastward, and 

 the lower westward. It is easy to see, however, that the former must prevail ; for 

 it has nothing to contend against but the friction of the latter, while the latter has 

 not only this same friction, but also that of the earth's surface, both conspiring to 

 destroy its motion westward. As a consequence, westerly winds must prevail in 

 this zone, though with less uniformity than exists in the other two.^ 



* I am aware that it may be urged as an objection to this view, that both the causes which are supposed 

 to determine the lower current toward the south-west, " operate with greater energy between the parallels 

 of 30° and 60°, than within the actual limits of the trade winds." (Mitchell.) But it should be noticed, 

 1st, that even if this were true, the causes which determine the upper current are increased in the same 

 ratio, and it is on the difference of the two only, that the mean direction depends. And 2d, that the objec- 

 tion itself is not well founded, so far as one of the causes is concerned; for, according to our views, the 

 zone of westerly winds lies wholly in the northern half of the vortex, where there could be no permanent 

 ascending currents, as at the equator, to create trade winds. 



It may be farther objected, that the theory here advocated requires an excess of northerly over southerly 

 winds in the temperate regions, so as to dispose of the current coming down from high northern latitudes. 

 We admit it, and are we sure that there is not? It is true that, in respect to time, the mean direction of the 

 wind in those parts of the zone that have been most fully studied (viz. the United States, Western Europe, 

 and the Atlantic Ocean), is from a point somewhat to the south of west. But are we sure that it is not 

 compensated by north-westerly winds over Asia and the North Pacific ? This is a question of fact, to be 

 determined by observation. Thus far the indications are, that there is such a compensation, and we await 

 with interest the results of Lieutenant Maury's investigations in that quarter of the globe to settle the 

 question. Furthermore, are we sure that even iu the parts of the zone first referred to, more air passes 

 northward than southward? On this point Professor Dove, of Berlin, has the following remarks, in a letter 

 to Colonel Sabine, published in the Report of the British Association for 1845: "But the air which 

 passes over the parallel, coming from the equator, brings with it a higher temperature, which it gradually 

 parts with as it flows over the surface of the earth, and which it cannot, therefore, bring back with it, 



