WINDS. 



371 



the 80° ; when it falls to 70°, the more rarefied space has receded to 

 1, which in its turn is reduced, the air following and flowing into 2 ; 

 thence in like manner the flow of air follows to 3 and 4. In this way 

 extensive tracts of country are swept over by the same winds, and 

 according to the difference of temperature will be the velocity of the 

 currents. 



As the land and sea breezes are familiar to all, and are well known 

 to be caused by the alternate heating and cooling of the atmosphere 

 over the land and sea, an illustration of their action, perhaps, will 

 make the progression more clearly understood : 



The shore, or dividing line, is that of the greatest heat. In the sea 

 breeze the flow is first from 1 to 0, which brings the to its tempera- 

 ture ; then recedes to 1, followed by the cooler air of ; and so on 

 to 2, 3, and 4, until the temperature of the land is reduced to the 

 temperature over the sea. Then, the same operation takes place 

 from the land, and the air over the sea attains a like temperature ; 

 at the changes, be it of short or long duration, a calm ensues. 



Before proceeding to the consideration of the subject of the winds 

 and vapors, I shall show what is the actual state of the earth's surface 

 as to temperature, the great cause, admitted by all, of the aerial 

 currents. 



For this purpose, I have divided the earth into five zones (see 

 annexed map), viz.: two frigid, two temperate, and one torrid zone. 

 The frigid zones are those from the Poles to the 60th parallel ; the 

 temperate zones are those between the 30th and 60th parallels, north 

 and south ; and the torrid zone embraces 30 degrees on each side of 

 the Equator, or the space between the Tropics. The extreme differ- 

 ence between these is more than 80° Fah. ; — that of the frigid zones 

 being considered as the seat of perpetual congelation, while that of the 

 torrid rises to a temperature above 80°. It is impossible to inspect 

 this map without being struck by the width of the belt of heated water 

 which surrounds the earth, comprising nearly the entire space within 



