METEOROLOG\ 3ld 



wliich, under various forms, may generate ague and typhus 

 i'ever (not by any means exclusively on wet, marshy ground, 

 or on coasts covered by putrescent mollusca, and low bushes 

 of Rhizophora mamgle and Avicennia). Fogs, which havo 

 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 heatin* 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 Ehrenherg's discovery, a host of siJicious-shelled in 

 lusoria 



As prnicipai features of a general descriptive picture of tlie 

 atmosphere, we may enumerate : 



1. Variations of atmospheric ^^^'cssure : to which belong 

 the horary oscillations, occurring with such regularity in the 

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

 mosphere, wliich 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 different 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 Composition Chimiqne dcs Vegetaux, p. 36, 42 ; Liebig, Org. Chemie, 

 6. 229-345; Boussingault, Econ. Rurale, t. i., p. 142-153. 



* Bouvard, by the application of the formula), iu 1827, which Laplaco 

 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 milH- 

 raeter, while eleven years' observations at the same place show the mean 

 barometric oscillation, from 9 A.M. to 3 P.M.. to be 756 millim., and 

 from 3 P.M. to 9 P.M., 0373 millim. See Min.oires de VAcad. den 

 Science-;, t. vii., 1827, p. 267. 



Vol. I.— O 



