2iO THE ATMOSPHERE AND METEOEOLOGY. 



A similar movement is produced on a very large scale in the equa- 

 torial regions. The great force of the sun's rays making themselves 

 principally felt in these countries of the world, the aerial strata expand 

 under the influence of the heat much more than in other latitudes. 

 They become lighter, and rise rapidly into space, as is shown by the 

 slight pressure of the air on the barometric column.* Thus a void 

 is formed, which the adjacent masses of air hasten to fill, and two 

 horizontal currents go to feed the great vertical current which ascends 

 towards the higher strata of air in the equatorial regions. But these 

 horizontal currents themselves leave spaces behind them towards 

 which new masses of air rush ; the atmospheric waves moVe nearer and 

 nearer through all the zones as far as the polar ice, and from the two 

 ends of the planet, they march towards the equator, where the ascend- 

 ing movement of the over-heated air summons them. Two winds, the 

 one from the north, the other from the south, take their origin from 

 the midst of the ice of the two opposite poles to meet at the equator- 

 ial circle. 



If the earth were not carried by a movement of rotation around its 

 axis, the atmospheric currents would flow directly towards the equa- 

 tor, without deviating to the right or left from the lines ofthe meri- 

 dian. The northern current would flow in a straight line to the south, 

 the southern current would direct itself exactly towards the north, 

 and they would meet in direct opposition in the equatorial regions. 

 But it does not happen thus, because of the rotation of the globe from 

 west to east. The speed of this movement varies for each point of the 

 terrestrial surface, as the diameter of its latitude ; whilst it is nothing 

 at the poles, it is 520 miles per hour at the 60th degree of latitude, 

 north or south, at the equator itself it is 1050 miles. The mass of air 

 which flows from the poles towards the tropical zone thus travels suc- 

 cessively over latitudes, whose own speed around the axis of the globe 

 is greater than theirs, consequently they are compelled to deviate 

 further and further towards the west in the opposite direction from 

 the general movement of the earth. Instead of being directed perpen- 

 dicularly towards the equator to form with it an angle of 90 degrees, 

 the aerial currents coming from the poles strike the equinoctial line 

 obliquely at an acute angle. Thus the same planetary phenomenon 

 that causes the deviation of the flow of water f of the oceanic currents, J 

 and perhaps even, according to M. Musset, the swelling of the trunks 

 of trees in the direction from east to west, sufiices likewise to put in 



* See above, p. 228. f See The Earth, tlie section entitled, Rivers. 



X See above, p. 92. 



