300 On the theory of the Variations of Atmospherical Pluenoinena. 



The explauation of the last-uamed phsenomenon, i. e. that of the 

 northern hemisphere, by a lateral overflow in the upper parts of 

 the atmosphere, seems so direct, that I think we may pronounce 

 the irregular form of the annual barometric curve in the West 

 Indies to be a secondary phaenomenon, the primary causes of 

 which must be looked for on the east. 



8. It is known that in the eruption of the Coseguina on the 

 20th of January, 1835, when the isthmus of Central America 

 was shaken by an earthquake, not only were volcanic ashes car- 

 ried to Kingston in Jamaica, a distance of 800 English miles ni 

 the opposite direction to the trade wind, but some of the same 

 ashes also fell 700 miles to the westward, on board the Conway, 

 in the Pacific Ocean. We infer, therefore, that in the higher 

 regions of the atmosphere in the tropics the air is not always 

 flowing regularly from S.W. to N.E., but that this usual and 

 regular direction is sometimes interrupted by currents from east 

 to west. I think I have indicated the probable cause of such 

 anomalous currents in the above described barometric relations 

 of the region of the monsoons compared with that of the trades. 

 If we suppose the upper portions of the air ascending over Asia 

 and Africa to flow off" laterally, and if this takes place suddenly, 

 it will check the course of the upper or counter current above 

 the trade wind, and force it to break into the lower current. An 

 east wind coming into a S.W. current must necessarily occasion 

 a rotatory movement, turning in the opposite direction to the 

 hands of a watch. A rotatory storm moving from S.E. to N.W. 

 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 rotation 

 (originating primarily in this manner) being that described by 

 me in detail in a memoir " On the Law of Storms,^' translated in 

 the Scientific Memoirs, vol. iii. art. 7. Thus it happens that 

 the West India hurricanes and the Chinese typhoons occur near 

 the lateral confines on either side of the great region of atmo- 

 spheric 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 monsoon 

 region, and being distinct from the storms appropriately called 

 by the Portuguese " Temporales,^' which accompany the out- 

 burst of the monsoon when the direction of the wind is reversed. 

 The fact of the rotatory storms being of much more rare occur- 

 rence in the South Atlantic Ocean arises from the more equal 

 distribution of the periodically diminished atmospheric pressure 

 in the southera as compared with the northern hemisphere. Here, 

 therefore, the rotatory storms take place principally in the mon- 

 soon itself. 



