METEOROLOGY 313 



which, under various forms, may generate ague and typhus 

 fever (not by any means exclusively on wet, marshy ground, 

 or on coasts covered by putrescent moUusca, and low bushes 

 of Rhizo'phoTa moMgle and Avicennia). Fogs, w^hich have 

 a peculiar smell at some seasons of the year, remind us of 

 these accidental admixtures in the lower strata of the atmos- 

 phere. Winds and currents of air caused by the heating of 

 the ground even carry up to a considerable elevation solid 

 substances reduced to a fine powder. The dust which dark- 

 ens the air for an extended area, and falls on the Cape Verd 

 Islands, to which Darwin has drawn attention, contains, ac- 

 cording to Ehrenberg's discovery, a host of silicious-shelled in 

 fusoria. 



As principal features of a general descriptive picture of the 

 atmosphere, we may enumerate : 



1 . Variations of atmospheric pressure : to which belong 

 the horary oscillations, occurring with such regularity in the 

 tropics, \vhere they produce a kind of ebb and flow in the at- 

 mosphere, wdiich can not be ascribed to the attraction of the 

 moon,^ and which differs so considerably according to geo- 

 graphical latitude, the seasons of the year, and the elevation 

 above the level of the sea. 



2. Climatic distribution of heat, which depends on the 

 relative position of the transparent and opaque masses (the 

 fluid and solid parts of the surface of the earth), and on the 

 hypsometrical configuration of continents ; relations which de- 

 termine the geographical position and curvature of the iso- 

 thermal lines (or curves of equal mean annual temperature) 

 both in a horizontal and vertical direction, or on a uniform 

 plane, or in difierent superposed strata of air. 



3. The distribution of the humidity of the atmosphere. 

 The quantitative relations of the humidity depend on the dif- 

 ferences in the solid and oceanic surfaces ; on the distance from 

 the equator and the level of the sea ; on the form in which the 



la ComposUion Chimi'que des Vegetaux, p. 36, 42 ; Liebig, Org. Cheinic. 

 s. 229-3-15; Boussiugaiilt, Eco7i. Rurale, t. i., p. 142-153. 



* Boavard, by the application of the formula), iu 1827, whicli La[)l:«ce 

 had deposited with the Board of Longitude shortly before his death, 

 found that the portion of the horary oscillations of the pressure of the 

 atmosphere, which depends on the attraction of the moon, can not raise 

 the mercury in the barometer at Paris more than the 0-018 of a milli- 

 meter, while eleven years' observations at the same place show the nieau 

 barometric oscillation, from 9 A.M. to 3 P.M.. to be 0756 millim., and 

 from 3 P.M. to 9 P.M., 0-373 millim. See M6moires de VAcad. des 

 Sciences, t. vii., 1827, p. 267. 



Vol. I.— O • 



