256 Mr. W. Brown on the Oscillations of the Barometer. 



are influenced by geographical situation, have their position, 

 with regard to colder, constantly on the equatorial side of 

 them, both in winter and summer; but in tropical regions 

 these positions in summer are reversed by the change in the 

 point over which the sun is vertical ; hence the currents also 

 are either permanently reversed during this season, as in the 

 monsoons of the Indian ocean ; or, as in the " trade winds," 

 their position is altered and their constancy interrupted, and 

 sometimes at least their direction reversed* ; thus the tropical 

 hurricanes of the Atlantic, which occur only during summer, 

 and the storms of the Indian ocean, which occur during the 

 same season, that is when the south-west monsoon is blowing, 

 have their relative parts precisely the reverse of those of high 

 latitudes, the descending current being from north, from which 

 quarter the storm commences, and the returning one from 

 south, whilst the progressive motion is towards north-west. 



These storms visit only the western parts of the Atlantic, a 

 fact which, on the supposition of their origin being the descent 

 of the upper current, is readily explained by a glance at the 

 position of the continents of Africa and America ; the western 

 part of the Atlantic having the latter stretching out on the 

 north of it and radiating the heat of the summer's sun, whilst 

 the former extends on the south of the eastern parts, having 

 only the waters of the ocean on the north. But these storms 

 near the boundary of the tropics change their directionf and 

 pass along the eastern coast of North America and parts ad- 

 jacent, and present phaenomena different from those of storms 

 which take their rise in extra-tropical latitudes, but to which 

 the explanation given in the foregoing paragraph of the phae- 

 nomena of the advancing portion of storms in the longitudes 

 of Europe, may with some modification be applied. In my 

 paper on the Storms of the Tropics, I have referred to two 

 storms of this kind, which advanced along the coast of the 

 United States in a direction from S.S.W. to N.N.E., of which 

 the data collected by W. C. Redfield are given in Col. Reid's 

 work on Storms. As a general explanation only was before 

 given of their phaenomena, it may now be proper to give one 



* For a particular explanation of the phaenomena of these storms I must 

 refer the reader to my essay in this Magazine, vol. xxiii. I may observe 

 here, however, that the identity of the phaenomena of the storms of the 

 Atlantic and Indian ocean is sufficient evidence of the same condition of 

 the currents in the former as in the latter at the time of the occurrence of 

 the hurricane; but in the western part of the Atlantic, which is the loca- 

 lity of the hurricanes, the south is the prevalent wind in summer. (See 

 an Essay on the Climate of Barbadoes, by Robert Lawson, in the Edin- 

 burgh Phil. Journal for July 1845.) 



f On the Storms of the Tropics, Phil. Mag. vol. xxiii. p. 206. 



