2J,S THE ATMOSPHEEE AND METEOEOLOGY. 



air, and strikes the surface of the water near the coasts of Portugal, 

 then turns back again, and makes itself successively felt at Madeira, 

 and on the middle and lower slopes of the peak of Teneriffe.* Accord- 

 ing to the astronomer Piazzi Smyth, it is at an average of 9000 feet 

 of vertical height that the plane of separation between the two aerial 

 rivers flowing in opposite directions is found. At the summit of the 

 mountain the air is carried rapidly from south-west to north-east, 

 while on the low parts of the island the trade-wind always blows 

 with its habitual regularity. f The zone of clouds that it unrolls in 

 an immense veil above the sea and the shores, does not extend into 

 that part of the heavens comprised between the two winds blowing 

 different ways, but, on the contrary, it is found at a tolerable depth 

 in the trade-winds. Between the upper and lower currents the 

 air is calm and free from clouds. During the summer season, 

 travellers who climb the sides of the peak of Teneriffe, may con- 

 fidently expect to find an unchanging sky directly after having 

 passed the zone of clouds, from 900 to 1200 feet in thickness, which 

 is spread like a second sea above the ocean. J At the change of 

 the seasons, when the two opposing winds strive for victory on the 

 slopes of the mountain, a few days are sometimes sufficient to bring 

 about a change of 3000 feet in the height of the intermediary 

 zone. A battle between the two currents takes place in the sky ; 

 soon the trade- wind mounts to the upper slopes of the peak ; now it 

 is vanquished, chased from the heights of the atmosphere, and driven 

 with all its system of clouds towards the lower regions. It is prin- 

 cipally above the pass of Laguna, between Santa- Cruz and Orotava, 

 that the combat takes place, and in consequence, this district of 

 the island is frequently inundated with rains. Piazzi Smyth has 

 recounted these grand aerial contests at great length in his work on 

 Teneriffe.§ 



In the Pacific Ocean, analogous phenomena to those which occur 

 in the Atlantic have been observed. Goodrich has ascertained that 

 the normal current of the trade and returning winds make themselves 

 felt at the same time, the one on the shores of the Sandwich Islands 

 and the lower slopes of all the mountains of the Archipelago, the 

 other on the summit of the volcano of Mauna-Loa. 



The direction of the upper counter-current is, like that of the 



♦ Humboldt ; Leopold von Buch, Description des Canaries. 



t PJiilosophical Transactions, 1859. 



X See below, the section entitled, Clouds and Rains. 



§ Teneriffe, pp. 178, 174, 432, &c. 



