THE ATMOSPHE11E. METEOROLOGY. 307 



and quaternary bases of the vegetable kingdom, ( 38 ) may 

 produce miasma which, under many forms and conditions 

 (and by no means exclusively on wet marshy grounds, or 

 on coasts covered with decaying mollusca and low bushes o( 

 mangrove and avicennia) may generate agues and typhus 

 fever. At certain seasons of the year, fogs having a peculiar 

 smell remind us of such adventitious and unwelcome 

 mixtures in the lower portions of the atmosphere. Winds 

 and ascending currents, caused by the heating of the 

 ground, may sometimes carry up even solid substances when 

 reduced to fine powder. The sand which creates an ob-. 

 scurity in the air over a wide area, and falls on the Cape 

 Yerd Islands, to which attention was directed by Darwin, 

 was found by Ehrenberg to contain innumerable siliceous- 

 shelled infusoria. 



As principal features of a general descriptive picture of 

 the atmosphere, we may distinguish 



1. Variations of atmospheric pressure ; those horary 

 fluctuations which within the tropics occur with such 

 regularity, and are so distinctly marked : they form a kind of 

 atmospheric tide, which, however, cannot be attributed to 

 the attraction of the Moon, ( 381 ) and which differs greatly 

 at different seasons, latitudes, and elevations. 



2. Climatic distribution of heat, dependent on the 

 relative position of transparent and opaque masses (the fluid 

 and solid portions of the surface of the globe), and on the 

 hypsometric configuration of continents. These determine 

 the geographical position and the curvature of the isothermal 

 lines (or lines of equal mean annual temperature), both in a 

 horizontal and a vertical direction, or on uniform or 

 different levels. 



