164 PHYSICUl, GEOGRAPHY OF THE SK\, AND ITS METEOROLOGY. 



cross — that from the southern passing over into the northern, and 

 that from the northern passing over into the southern hemisphere 

 (see Q E S, and D E F Gr, § 215) — yet some have imphed 

 doubt by asking the question, " How are two such cm-rents of air 

 to pass each other ?" And, for the want of light upon this point, 

 the correctness of my reasoning, facts, inferences, and deductions 

 have been questioned. In the fii'st place, it may be said in reply, 

 the belt of equatorial calms is often several hundred miles across, 

 seldom less than sixty ; whereas the depth of the volume of air that 

 the trade-winds pom- into it is only about three miles, for that is 

 supposed to be about the height to which the trade-wmds extend. 

 Thus we have the air passing into these calms by an opening on 

 the north side for the north-east trades, and another on the south 

 for the south-east trades, having a cross section of tln^ee miles ver- 

 tically to each opening. It then escapes by an opening upward, 

 the cross section of which is sixty or one himdred, or even three 

 hmidred miles. A very slow motion upward there will cany off 

 the air in that direction as fast as the two systems of trade-winds, 

 with their motion of twenty miles an hour, can pour it in ; and that 

 curds or flahes of aii' can readily cross each other and pass in 

 different directions without interfering the one with the other, or 

 at least without interfering to that degree which prevents, we all 

 know. The brown fields in summer afford evidence in a striking 

 manner of the fact that, in nature, flakes, or streamlets, or ciu'dles 

 of air do really move among each other without obstruction. 

 That tremulous motion which we so often observe above stubble- 

 fields, barren wastes, or above any heated smface, is caused by the 

 ascent and descent, at one and the same time, of flakes of air at 

 different temperatm^es, the cool coming dow^i, the warm going up. 

 They do not readily commingle, for the astronomer long after 

 nightfall, when he tm^ns his telescope upon the heavens, perceives 

 and laments the unsteadiness they produce in the sky. If the air 

 brought to the calm belt by the north-east trade-winds difter in 

 temperature (and why not ?) from that brought by the south-east 

 trades we have the authority of nature for saying that the two 

 currents would not readily commingle (§ 98). Proof is daily 

 afforded that they would not, and there is reason to believe that the 

 air of each cm-rent, in streaks, or patches, or flahes, does thread its 

 way through the air of the other without difliculty. Therefore we 

 may assume it as a postulate which natm-e concedes, that there is no 

 physical difficidty as to the two cm-rents of air, which come into 



