COUNTER TEADE- WINDS. 245 



CHAPTEE Y. 



COUNTER TRADE-WINDS OR RETURNING WINDS. 



The aerial masses brought by the two trade-winds cannot be in- 

 cessantly accumulated in the region of equatorial calms ; they expand, 

 rise to several miles of height, then, after having been mixed, and 

 even partially crossed, they divide anew into two great returning 

 currents which flow in an opposite direction in the upper regions of 

 the atmosphere. Thus, as the natural philosopher Halley, who was 

 the first to give a theory of the trade- winds, affirmed nearly two 

 centuries ago, it would be absolutely impossible, if these two atmo- 

 spheric counter-currents did not exist, that the equilibrium of the air 

 could be established on the surface of the globe ; that which the 

 breath of the trade- winds bring to the equator must necessarily be 

 carried back by other winds towards the poles. The movement of 

 the graceful clouds, so light that we see them from below floating in 

 the heights of the air in the opposite direction to the trade-winds, is 

 an incontestable proof of the existence of these higher returning 

 currents.* Besides, two great volcanic explosions, often mentioned by 

 savants, have also furnished striking testimonies which confirm 

 Halley 's theory in an indubitable manner. On the 1st of May, 1812, 

 when the north-east trade-wind was in all its force, enormous quan- 

 tities of ashes obscured the atmosphere above the island of Barba- 

 does, and covered the ground with a thick layer. From w^hence came 

 these clouds of dust ?' One would have supposed that they came from 

 the volcanos of the Azores, which were to the north-east, never- 

 theless, they were cast up by the crater of Morne Garou, situated in 

 the island of St Vincent, at 125 miles to the west. It is therefore 

 certain that the debris had been hurled by the force of the eruption 

 above the moving sheet of the trade-winds, into an aerial river, pro- 

 ceeding in the contrary direction. In the same way, at the time of 

 the terrible eruption of the volcano of Coseguina, in Central America, 

 ashes were carried by the returning trade-winds to the shores of 

 * See The Earth, the section entitled, Volcanos. 



