228 METEOROLOGY. 



into the valleys of Aragua. This breeze rises regularly two or three 

 hours after sunset." 



Now, here we see that this phenomenon was as regularly exhibited 

 fifty-seven years ago as it is now, and no doubt has been going on 

 from time immemorial. The same causes that were then at work are 

 still at work as forcibly as ever. While at Valencia the slapping of 

 the doors, the columns of dust that came sweeping by, and other | 

 signs of a violent rush of wind indicated to me with unfailing cer- .| 

 tamty that it was after 3 or 4 p. m. Travellers and muleteers, in ij 

 going from Valencia to Puerto Cabello, are especially annoyed on the i\ 

 first six miles of their road by the dust, so that they have to cover • 

 their faces or shut their eyes most of the time. 



It was natural for me to reflect upon the probable cause of this phe- • 

 nomenon, and a circumstance soon presented itself to help me towards i 

 the solution of this problem. 



For on the 11th of March the sky was densely clouded, all day, over ■ 

 Valencia, and the whole valley of Aragua, including the lake, and it 

 looked as if it was going to rain. In the evening, the usual regular • 

 wind failed to make its appearance altogether. 



To account for the alternating daily land and sea breezes of the ; 

 coasts in general, there exists a well-known explanation, based upon 

 the rarefaction of the air by the heated surface of the land or water. 

 This explanation, however, will not do in the above case ; for at Va- 

 lencia the wind begins to blow from the seaside not in the niorning, 

 when the land becomes heated, but, on the contrary, in the afternoon 

 when the land begins to cool again. 



When in equatorial regions the direct rays of the sun act for some ' 

 time upon a widely spread mass of water, surrounded by land, as, for 

 instance, a lake, they evaporate powerfully the water from its surface ; 

 that is, they convert a liquid into an aeriform fluid. In this latter 

 state the water requires several hundred times more room and exerts a 

 certain pressure which added to the pressure of the atmosphere,* with 

 which this vapor is mixed, overpowers the surrounding dryer atmos- 

 phere, and spreads or shoves the latter outward in nil directions to ) 

 make room for itself. In such an atmosphere, saturated with invisible : 

 vapor, its permanent gases are much more attenuated than when im 

 a dry state ; but they nevertheless exert, by the aid of the invisible ■ 

 vapor, an overwhelming outward pressure. 



That amount of pressure, however, which tlie vapor exerts, can be 

 easily annihilated by condensation, and thereby a partial vacuum be 

 produced, into which the external air will strive to rush with great I 



force. 



Now, in the surrounding bottom lands of the lake, the air loses 

 towards evening, in the dry season, by radiation against an unclouded I 

 sky, more caloric than it receives. 



It therefore seems highly probable, that the gradual decrease of' 

 temperature, by which the vapors of the air above the lake of Valencia 

 lose part of their tension, is the cause of the r egular breeze which 



* Accordino- to the great fundamental principle, that in any given space, the power of ten- 

 sion of two or more mixed aeritorm bodies is equal to the sum of all those tensions which 

 each of these aerial bodies would exert, if it was to occupy the whole space by itselt. 



