144 SCIENTIFIC AGRICULTURE. 



by night, only the current is reversed." At the time when a 

 perfect equilibrium exists between the temperature of the land 

 and sea, the wind ceases, and there is for a time a dead calm. 



Trade winds are produced by the same causes operating 

 upon a larger scale, and the revolution of the earth on its own 

 axis. These are tropical winds, which prevail only within the 

 limit of about 30 each side of the equator. Their general 

 course on the north side of the equator, is from north-east to 

 south-west, and on the south side, from south-east to north- 

 west. The upward current of the air at the equator, in conse- 

 quence of its higher temperature, causes the colder air to rush 

 in from the north and south towards the point of greater rare- 

 faction; this produces the northward and southward currents. 

 These currents have now a westerly tendency given them by 

 the diurnal rotation of the earth on its axis towards the cast; 

 thus producing their general directions as above described. 

 When not changed by local causes, their direction is the same 

 throughout the year : but however they may be modified, they 

 always blow towards the point of greatest rarefaction, and 

 receive a relative motion from the earth's diurnal revolution. 

 Their velocity is greatest at the equator, where the earth's 

 motion is the most rapid, and diminishes towards the poles in 

 proportion as the circumference of the earth diminishes, and 

 the motion is less rapid. 



The harmattan wind is a periodical easterly wind, which 

 olows irregulary in Africa: it occurs three or four times 

 yearly, and continues for a longer or shorter period, according 

 to circumstances. It blows with only a moderate velocity, 

 is peculiarly dry and unpleasant : it is attended by a haziness 

 of the atmosphere, which often obscures the sun most of the 

 day. During this wind there is no moisture in the air, and no 

 dew or fog; vegetation becomes parched, and droops. Not- 

 withstanding the depressing and disagreeable effects of the 

 harmattan, it is said to be a salubrious wind. 



