ACADEMY OF SCIENCES. Ill 



may be well to recall to mind tbe more general laws which govern the atmos- 

 pheric movements — laws which are as invariable and determinate in their action 

 as the ordinary changes of the weather are apparently capricious. It is well 

 known that a current, or currents, of cold air are always flowing from the 

 colder regions of the earth towards the Equator, and currents of heated air are 

 as constantly flowing from the region of the Equator towards the Poles. At 

 from 25° to 30° of the Equator the Polar currents are found spread out over 

 the whole surface of the ocean, constituting the northeast Trades to the north 

 of the Equator and the southeast Trades in the Southern Hemisphere. These 

 two vast bodies of air that are constantly flowing towards the Equator from 

 the Poles, are separated from each other near the Equator by a belt known as 

 the region of calms. There, there is hardly any horizontal movement in the at- 

 mosphere. The air, now heated and loaded with moisture, which has been 

 flowing from each side towards this point, ascends into the upper region of the 

 atmosphere, and, as the southwest anti-trade, commences its journey again 

 towards the Poles. Thus a constant movement of currents in opposite direc- 

 tions is for ever going on in the atmosphere. In the region of the Trade 

 Winds these two currents pursue their course in a quiet sort of way — the Polar 

 current below ,the Equatorial current above — rarely interfering with each other. 

 When, however, we pass beyond the region of the Trades, these currents no 

 longer occupy the same relative positions. The warm moist current as it flows 

 into higher latitudes finds itself compressed laterally on account of the smaller 

 circles of longitude, and the Polar current has to spread itself out horizontally 

 in order to cover a larger surface as it approaches the Equator. From this 

 cause, and possibly for other reasons which are not discovered, the upper current 

 now comes down to the surface in certain points, forming as it were a channel 

 for itself through diSerent parts of the Polar current. The two currents do 

 not mingle indiscriminately. There seem to be large continuous portions of the 

 earth's surface which are occupied by one current, whilst running parallel, 

 although in an opposite direction, is a broad current which occupies another 

 large portion of the surface. The breadth of these currents and the points they 

 usually occupy on the surface of the earth are data which are still undeter- 

 mined, but it has been found as a general rule that the cold, or Polar currents 

 prevail more in the interior of the larger continents, while the Equatorial cur- 

 rents mostly reach the Poles over the surface of the larger oceans and the 

 lands near the coasts. There can be no doubt that the climate of any country 

 is most essentially influenced by the position it occupies as regards these vast 

 streams of atmosphere — places situated within the Polar stream enjoying a 

 dry climate and those in the Equatorial stream a moist climate — and did these 

 streams always occupy the same positions on the earth's surface, the dryness, 

 or humidity of a place would depend on its geographical position. But these 

 two currents, although well marked in their general boundaries, by no means 

 pursue their course without influencing each other, on the one hand and on the 

 other, without being strongly affected by tbe orological features of the regions 

 they traverse. It is not here the place to enter into the minutiaj of these 

 phenomena, even as far as they are now known, and up to the present time 



