130 Annual Variation of Atmospheric Pressure. 



course of the upper or counter current above the trade-wind, 

 and breaks into the lower current. An east wind coming 

 into a SW. current must necessarily occasion a rotatory 

 movement, turning in the opposite direction to the hands of 

 a watch. A rotatory storm moving from SE. to NW. in the 

 lower current or trade would, in this view, be the result of the 

 encounter of two masses of air impelled towards each other 

 at many places in succession, the further course of the rota- 

 tion (originating primarily in this manner) being that de- 

 scribed by me in detail in a memoir on " the Law of Storms,'' 

 translated in the Scientific Memoirs, vol. iii., art. viii. 

 Thus it happens that the West India hurricanes and the 

 Chinese typhoons occur near lateral confines on either side of 

 the great region of atmospheric expansion ; the typhoons being 

 probably occasioned by the direct pressure of the air from the 

 region of the trade-winds over the Pacific into the more 

 expanded air of the monsoons region, and being distinct from 

 the storms appropriately called by the Portuguese "Tem- 

 porals," which accompany the outburst of the monsoon when 

 the direction of the wind is reserved. The fact of the 

 rotatory storms being of much more rare occurrence in the 

 South Atlantic Ocean arises from the more equal distribution 

 of the periodically diminished atmospheric pressure in the 

 southern as compared with the northern hemisphere. Here, 

 therefore, the rotatory storms take place principally in the 

 monsoon itself. 



9. It is evident that the unsymmetrical distribution of land 

 and sea, which gives rise to the abnormal variations in the 

 forms of the isothermal lines, is at the same time the principal 

 cause of the movements of the atmosphere. Thus the mon- 

 soon is but a modification of the trade-wind, of which the 

 cause is to be sought in part beyond the tropic. The region 

 of great thermic expansion of the air in summer in the 

 interior of the continent of the Old World presents all the 

 characteristic marks of the region of calms, being a centre 

 towards which all adjacent masses of air are drawn. Hence 

 there is no complete sub-tropical zone, in the sense of a zone 

 encompassing the globe. The region over which the heated 

 air ascends does not therefore move up and down, or north 



