Atmospherical Phenomena. 251 



For a long time the cause of this wind was unknown, until at 

 length it was shewn by Halley to depend on the difference of 

 temperature between lower and higher latitudes. 



The difference of temperature between the equinoctial re- 

 gions and those of higher latitudes, causes, in the upper parts 

 of the atmosphere, currents of air from the equator to the poles, 

 while, near the surface, these currents have an opposite di- 

 rection. If the earth were a motionless body, then, on the sur- 

 face of the sea, where there are no mountains or other similar ob- 

 structions presenting obstacles to the air currents, north winds 

 would blow in one hemisphere, and south winds in the opposite ; 

 but this direction is somewhat altered by the rotation of the earth. 

 For when the masses of air approach the equator, they do not 

 immediately participate in the more rapid movement of the re- 

 gions which they reach ; and as they remain behind, they op- 

 pose an obstacle on the east side of the bodies which occur on 

 the surface, and which are moving from west to east. By the 

 union of this direction with that coming from the pole, we get 

 a north-east wind for the northern hemisphere, and a south- 

 east wind for the southern hemisphere. But the motion of the 

 earth has an influence not only on the lower currents of air, 

 but also on the upper equatorial current ; for when this reaches 

 higher latitudes, it comes into regions in which the rapidity 

 of rotation is less ; hence the current of air hastens on before 

 the earth"'s surface, and we thus meet with a south-west wind 

 in the northern hemisphere, and a north-west wind in the 

 southern. 



The facts which have been communicated by navigators for 

 centuries, confirm these theoretical conclusions ; and -we find 

 not merely the trade-winds as the theory requires, but we also 

 find that the intermediate space, by which the south-east and 

 north-east trade-winds are separated, changes simultaneously 

 with the sun from north to south. Although, in the regions 

 where the trade-wind blows with regularity, the sky is almost 

 always serene, so that it is difficult to determine the condition 

 of the upper current of air by means of the course of the 

 clouds, yet we see, nevertheless, that the light small clouds, 

 which occasionally appear in the upper strata of the atmosphere, 

 for the most part move against the trade-wind ; thus, on the 

 ummit of the peak of Teneriffe, a more or less strong westerly 



