THE SMITHSONIAN INSTITUTION. 187 



This is a very simple principle, but it is the onl}^ one which enables 

 us to comprehend how a dead calm often exists immediately before 

 violent storms, and even before the tremendous hurricanes of the West 

 India ishinds. A large amount of power is in this wa}^ held in reserve 

 ready to be developed under various circumstances. We shall men- 

 tion some phenomena as illustrations which are produced in this way. 



The unstable condition of the air, which results from the undue heat 

 of the lower stratum, produces those great whirlwinds of dust and sand 

 in the deserts of Arabia and Africa. The air flows in beneath, and 

 revolves as it ascends, carrying loose material with it. Humboldt, 

 when crossing over the great plains of South America during the hot 

 season, recorded a curious instance of the effect of the sun's rays en the 

 surface of the ground when the air was calm : "In the Mesa de Paja," 

 says that illustrious traveller, " we eiUered the basin of the Llanos. The 

 sun was almost at its zenith ; the earth, wherever it appeared sterile 

 and destitute of vegetation, was at the temperature of 86^ to 90^ F.; 

 not a breath of wind was felt at the height at which we were on our mules ; 

 yet in the midst of this apparent calm whirls of dust incessantl}^ arose, 

 driven on by these small currents of air which glide only over the sur- 

 face of the ground, and are occasioned by the difference of temperature 

 between the naked sand and the spots covered with grass." — (Personal 

 Narrative, Vol. 1.) The land and the sea breezes observed in warm 

 countries are caused by somewhat similar agencies. The land in the 

 day time becomes heated to a much greater degree than the sea ; the 

 air over the former expands and flows away above, while the compara- 

 tively cold air from the sea rushes over the land. This is exactly 

 reversed at night, the air over the sea being slightly warmer than over 

 the land — the breeze is from the latter. All are agreed as to the 

 general cause of this phenomenon, but I am inclined to think the par- 

 ticular mode of action has not received that attention which it deserves. 

 As I shall have occasion to show that the action of the sea breeze in 

 Great Britain apparently furnishes us with the true principle upon 

 which certain violent disturbances sometimes take place over one halt 

 of Europe, I shall be a little particular in describing the exact mode 

 of action. x\nd I am not singular in opinion that a modification ot the 

 same principle applies to some of the storms of the United Slates. 

 On a recent visit to Cuba I had an opportunity to study some well 

 defined instances of land and sea breezes. I was particularly in- 

 terested in the former, because I was less familiar with them from personal 

 observation ; for while the sea breeze is common in summer on the 

 east coast of Scotland, the land breeze is very rare. Hugh Miller has 

 given a description of the sea breeze in his work entitled "My School 

 and Schoolmasters," as it occurs on the Cromart}'- coast, which is well 

 worth a perusal by those who take an interest in such matters. 



The particular summer sea breeze of North Britain, so far as I have 

 had an opportunity of observing, only occurs when an upjx-r current 

 is flowing from a westerly quarter. This condition seems to be essen- 

 tial to its action. The breeze is always strongest on the coast, it gradu- 

 ally moderates as it passes into the interior, and final.ly dies away long 

 before it reaches the west coast. The mauner in which the sea breeze 

 loses its force shows that it is graduall}/ absorbed into the upper current, 



