136 THE TRADE-WINDS. [SECT. xv. 



its undulation. Since each oscillation has its perfect effect 

 independently of the others, each one is marked by a change 

 in the barometer, and this is beautifully illustrated by 

 curves constructed from a series of observations. The 

 general form of the curve shows the course of the principal 

 wave, while small undulations in its outline mark the 

 maxima and minima of the minor oscillations. 



The trade-winds, which are the principal currents in the 

 atmosphere, are only a particular case of those very general 

 laws which regulate the motion of the winds depending on 

 the rarefaction of the air combined with the rotation of the 

 earth on its axis. 



The heat of the sun occasions these aerial currents by 

 rarefying the air at the equator, which causes the cooler and 

 more dense part of the atmosphere to rush along the surface 

 of the earth from the poles towards the equator, while that 

 which is heated is carried along the higher strata to the poles, 

 forming two counter-currents in the direction of the meridian. 

 But the rotatory velocity of the air corresponding to its geo- 

 graphical position decreases towards the poles. In approach- 

 ing the equator it must therefore revolve more slowly than 

 the corresponding parts of the earth, and the bodies on the 

 surface of the earth must strike against it with the excess of 

 their velocity, and by its reaction they will meet with a 

 resistance contrary to their motion of rotation. So that the 

 wind will appear, to a person supposing himself to be at rest, 

 to blow in a direction nearly though not altogether contrary 

 to the earth's rotation ; because these currents will still 

 retain a part of their northerly and southerly impetus, which, 

 combining with their deficiency of rotatory velocity, will 

 make them appear to blow from the north-east on one side 

 of the equator and from the south-east on the other, which 

 is the general direction of the trade-winds. But they are 

 modified both in intensity and direction by the seasons, by 

 the neighbourhood of continents, and by the nature of the 

 soil, so that the phenomena are not the same in both hemi- 



