116 THE TRADE;- WINDS. SBCT. xv. 



and others thirty-six hours to accomplish their rise and 

 fall. One especially of these vast barometric waves, many 

 hundreds of miles in breadth, has been traced over the 

 greater part of Europe, and not its breadth only, but also 

 the direction of its front and its velocity have been clearly 

 ascertained. Although like ah 1 other waves these are 

 but moving forms, yet winds arise dependent on them 

 like tide streams in the ocean. Mr. Birt has deter- 

 mined the periods of other waves of still greater extent 

 and duration, two of which require seventeen days to 

 rise and fall, and another took thirteen days to complete 

 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 obser- 

 vations. 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 ae'rial 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 toward 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 ve- 

 locity of the air corresponding to its geographical posi- 

 tion decreases toward the poles. In approaching 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 ex- 

 cess 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 him- 

 self 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 



