216 



THE ATMOSPHEEE AND METEOEOLOGT. 



Jamaica, whicli is no less than 800 miles to the north-east of the 

 crater whence they were thrown. 





Fig. 103.— Cloud of ciuders from Morne Garou. 



On the coasts of Africa, and along the Mediterranean, grains of 

 dust, almost imperceptible singly, give another very remarkable proof 

 of the existence of a great returning current in the high regions of 

 the atmosphere. Sometimes a shower of yellow or red dust, resembling 

 powdered brick, falls from the sky. Ships which were in the neigh- 

 bourhood of Cape Yerde, on the coasts of Morocco, or in the waters 

 of the Mediterranean, have had their deck and sails completely 

 sprinkled with these fine particles. Humboldt, who had the opportu- 

 nity to observe this rain, believed that it was composed of silicious dust 

 raised by eddies of wind on the coasts of the Sahara, while the sailors 

 who witnessed this phenomenon saw in it only a shower of sulphur. 

 But Ehrenberg, with the aid of his microscope, revealed the nature of 

 this dust, which is nothing else, at least in the Atlantic and the 

 Mediterranean, than the silicious skeletons of animalculae coming from 

 the llanos of South America. It is thus certain that these myriads 

 of organisms,, raised to a height in the air by the ascending current of 

 the equator, have met above the trade- wiuds with a returning cur- 

 rent, which has caused them to cross the immense basin of the Atlan- 

 tic, and to reach the coasts of Africa, or even of Europe, as far as the 



