Mr. \V. Brown on the Storms of Tropical Latitudes. 279 



of tropical hurricanes, by checking the acceleration of the 

 southern or upper current. 



The differences of the two descriptions of storms, with re- 

 gard to the fall of the barometer and the extent of each parti- 

 cular portion of the hurricane,are well compared by Professor 

 Dove to those between a deep ravine with precipitous sides, 

 and an extensive valley with gentle declivities ; a comparison 

 however holding much more truly with regard to the views 

 here taken, than to those adopted by Professor Dove in ad- 

 vocating the hypothesis of a whirlwind*. 



It will be easily seen that these differences arise from the 

 difference in the force of the wind. The more violent the wind 

 the more rapid will be the fall of the barometer, but at the 

 same time the resistance to the force of the wind will increase 

 more rapidly also, and overcome it in less time than when the 

 wind is less violent. 



The remaining differences are obviously referable to the same 

 origin as those generally existing between the meteorological 

 phenomena of these two divisions of the globe, — the great 

 complexity of the causes which affect the distribution of heat 

 and atmospheric vapour in temperate regions as compared 

 with those of the tropics. Thus whilst in the latter each storm 

 is generally one individual phasnomenon, the former are fre- 

 quently visited by them in rapid succession, each succeeding 

 one appearing before the subsidence of the last, and thus 

 often preventing the regularityof the succession of the currents, 

 here described as belonging to tropical gales and sometimes 

 to these, being observed. 



An interesting observation regarding the locality of hurri- 

 canes has been made by Col. Reid, which is the fact of the 

 exemption from them of the region adjacent to St. Helena, 

 whose position occurs on the line of least magnetic intensity, 

 as drawn on Major Sabine's chart ; not however as Col. Reid 

 appears to suggest, as pointing out the existence of a force 

 not previously recognised as at work in the movements of the 

 atmosphere, but as indicating the connexion between the lo- 

 calities of storms and climate; Sir David Brewster having 

 shown the relation existing between the isodynamic magnetic 

 lines and the isothermal lines; both these series of lines 

 agreeing in general in their deviations from the circles of la- 

 titudes, the lines of greatest magnetic intensity following those 

 of greatest heat. The dependence of the force of the wind on 

 the relative difference of temperature of adjacent latitudes, has 

 been before shown with regard to seasons; storms will be 



* Dove on the Law of Storms (Taylor's Scientific Memoirs, vol. iii. 

 (part x.) p. 217). 



