in different parts of the Globe. 129 



considerable ; and it is generally greater in the northern 

 than in the corresponding southern latitudes. The exact 

 contrary is the case on the Atlantic side and in the region 

 of the Trades ; for here the annual variation, though no- 

 where very considerable, is decidedly greater in the southern 

 than in the northern hemisphere, as is shewn by the results 

 of observation at -the Cape, Ascension, St Helena, Rio 

 Janeiro, and Pernambuco, compared with the West Indian 

 Islands and the southern parts of the United States. Hence 

 it follows that, if we compare places in the same latitude, 

 we find but little difference between the annual variation in 

 the southern Atlantic and southern Indian oceans, while in 

 the northern hemisphere we have in the same latitude the 

 very large annual variation in the north part of the Indian 

 and in the Chinese seas, and the almost entire absence of 

 annual variation in the Atlantic (compare Chusan with the 

 Azores and Madeira). The explanation of the last-named 

 phenomenon, 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 phenomenon, 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 the volcanic 

 ashes carried to Kingston in Jamaica, a distance of 800 

 English miles in the opposite direction to the trade-wind, but 

 some of the same ashes 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 SW. to NE., 

 but that this usual and regular direction is sometimes inter- 

 rupted 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 checks the 



VOL. LV. NO. CIX. — JULY 1853. 1 



