[ 



ALTITUDE 01^ EETUEN CUEEENT. 



247' 



basin of tlie Ehone. The aerial currrents have thus become visible, 

 by means of these clouds of infusoria.* 



In the equatorial zone the counter- current of the trade-winds can 

 only commence at a height of from four to five miles above the level 

 of the sea, for the highest summits of the Cordilleras remain entirely 

 bathed in the lower current. The most southern mountain of the 

 Atlantic basin, whence the returning wind has been observed, is the 

 peak of Teyde, in the island of Teneriffe. There the masses of air 



Fig. 104.— Island of Teneriffe. 



flowing back from the equatorial zone, are already sufficiently low to 

 surround the terminal point of the volcano (only 12,060 feet high) at 

 all seasons. In winter, when the whole circulatory sj^stem of the 

 atmosphere has descended towards the south, following the course of 

 the sun, the returning current descends from the higher strata of the 

 * Maury, Physical Geography of the Sea. 



