27d Mr. W. Brown on the Storms of Tropical Latitudes. 



fent We have ( ( t^\ x 12 5 = 0°-2Gl. The product of this 



by | (on account of the pulsatory character of the magneto- 

 current) gives o, 34-8, which, as theory demands, is somewhat 

 less than the quantity found by experiment. 

 [To be continued.] 



XXXIII. 0?i the Storms of Tropical Latitudes. By William 



Brown, Jan. 



[Continued from p. 217 and concluded.] 



''l^HE storms of the tropics and of temperate regions, though 

 thus referable to the same source, have yet some marked dif- 

 ferences. The principal of these are : — the greater violence of 

 the former ; the isolation of each individual storm ; the less 

 extent of each particular portion of it, and the much greater 

 extent and regularity of its progressive motion ; and the more 

 rapid though less depression of the barometer. 



It may seem difficult at first to account for the violence of 

 tropical hurricanes, but the difficulty disappears when we 

 cease to compare the force of the upper current with that of 

 the lower which sweeps oyer the earth, where the friction upon 

 the surface prevents its gaining a great velocity by soon putting 

 alimit to its acceleration. But as at the beginning of the hurri- 

 canes which arise in the region of the trade-wind the lower cur- 

 rent has not effected the change corresponding to that of the 

 upper one, both therefore flowing in the same direction, the effect 

 of friction must be so slight that we may almost disregard it ; 

 as we may do also in other portions of the zone of the tropics, 

 where a permanent reversal of the currents is effected; be- 

 cause it must be supposed that the upper and lower currents 

 of the atmosphere are separated from each other by an in- 

 terval of calm air; therefore the upper current when confined 

 to the upper strata of the atmosphere, subject as it is from 

 the constant decrease in temperature it meets with to an ac- 

 celerating force, may attain a very great velocity, more espe- 

 cially in the former regions, before the effect of friction is 

 sufficiently powerful to put a limit to its acceleration. 



Thus as it matters little whether the difference of tempera- 

 ture, by which the wind acquires force sufficient to give the 

 velocity of a hurricane notwithstanding the resistance of the 

 air which it must displace, exists between columns of air 

 nearly adjacent, or at the distance of many degrees of lati- 

 tude, there seems no difficulty in conceiving the difference to 

 be sufficiently great. 



The comparative exemption of the upper current from fric- 



