Facts in Meteorology. 12^ 



south west, and on the peak of TenerlfFe, about ten thousand feet 

 above the sea, a strong wind blows from the west. 



Volcanic ashes when carried into the higher regions of the atnios- 

 phere are usually wafted to the eastward. Upon an eruption of 

 Mount Vesuvius in 1631, a shower of ashes fell upon the coast of 

 continental Greece, and also at one hundred leagues distance to- 

 wards the coast of Syria. On the eruption at St. Vincent in 1812, 

 ashes were deposited at Barbadoes, sixty or seventy miles eastward, 

 and also on the decks of vessels one hundred miles still farther east, 

 while the trade wind at the surface was blowing in its usual direction. 

 In the same year ashes fell upon the deck of a British packet bound 

 to Brazil, when distant nearly one thousand miles from the nearest 

 land. 



Nearly all the sensible phenomena of the atmosphere occur be- 

 low the height of eighteen thousand feet, and generally much nearer 

 to the earth's surface. Owing to the retardation of the surface winds 

 and to other causes, the currents in the lower atmosphere run upon 

 each other in horizontal strata, which differ much in temperature 

 and hygrometrical condition, as well as in the direction and velocity 

 of movement. It is chiefly to the condition of these strata and their 

 influence upon each other that the formation of clouds and rain is 

 to be ascribed. 



Of the Barometer. 



The fluctuations of the barometer appear to be owing to different 

 causes, and may be classed under the three following heads : 



1. The regular semi-diurnal variation, which, in the tropical lati- 

 tudes, is at its maximum about 10 A. M., and at its minimum about 

 3 P. M. At New York it is nearly the same, but at Edinburgh the 

 effect is reversed, the minimum being at 10 and the maximum at 

 3 o'clock. It appears to indicate a system of atmospheric tides, re- 

 sulting from the rotation of the earth and its connexion with the so- 

 lar system. 



2. The variations resulting from the mechanical action of circuit- 

 ous winds and the larger atmospheric eddies ; including not only the 

 storms but a large portion of the winds in the higher latitudes. 

 During the passage of these eddies or storms over the place of ob- 

 servation, the barometer sinks while under their first or most advan- 

 ced portions, and rises as they pass over or recede. The most prom- 

 inent variations of the barometer are of this character. 



Vol. XXV.— No. 1. 17 



