METEOEOLOGY. 319 



of the atmosphere. 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 

 darkens the air for an extended area and falls on the Cape 

 Verd Islands, to which Darwin has drawn attention, contains, 

 according to Ehrenberg's discovery, a host of siliceous shelled 

 infusoria. 



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

 atmosphere, w^hich cannot be ascribed to the attraction of the 

 moon,* and which differs so considerably according to geogra- 

 phical latitude, the seasons of the year, and the elevation above 

 the level of the sea. 



2. Climatic distribution of heat, w r hich 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 hypsome- 

 trical configuration of continents relations which determine 

 the geographical position and curvature of the isothermal 

 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 differ- 

 ences in the solid and oceanic surfaces on the distance from 

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

 aqueous vapour is precipitated, and on the connection exist- 

 ing between these deposits and the changes of temperature, 

 and the direction and succession of winds. 



"~4. The electric condition of the atmosphere. The primary 

 cause of this condition, when the heavens are serene, is still 



* Bouvard, by the application of the formulae, in 1827, which Laplace 

 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, cannot raise 

 the mercury in the barometer at Paris more than the O'OIS of a milli- 

 metre ; whilst eleven years' observations at the same place show the 

 mean barometric oscillation, from 9 A.M. to 3 P.M., to be 0*756 millim., 

 and from 3 P.M. to 9 P.M., 0'373 millim. ; see Memoires de I'Acad. des 

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



