278 Mr. W. Brown on the Storms of Tropical Latitudes. 



That this is the true explanation of this variation can scarcely, 

 I think, admit of doubt ; it requires of course that the press- 

 ure of the atmosphere near the southern confines of the mon- 

 soons should vary simultaneously with that of the northern, 

 but in the opposite direction, and that at the equator, or mid- 

 way in the course of each monsoon, it should remain nearly 

 stationary throughout the year; and also that the variation 

 should be general throughout the zone of the tropics, because 

 although the atmospheric currents in only some portions of it 

 have the regular alternations of monsoons, they are everywhere 

 more or less influenced both in force and direction by the 

 movement of the sun in declination. I am not aware of any 

 observations in the longitudes of India or China to enable 

 us to answer the first of these questions with regard to the 

 monsoons themselves, but the occurrence of the variations in 

 the required order in the longitudes of the New World is 

 shown by the observations given by Humboldt in his " Perso- 

 nal Narrative;" the results of which with regard to this sub- 

 ject, he thus states (English edit., vol. vi. p. 747) : — " It is in- 

 teresting to compare the variations of the weight of the atmo- 

 sphere in the vicinity of the two tropics. At Rio Janeiro (south 

 latitude 23° 30') the extreme barometric mean of December 

 and August, and at the Havannah (north latitude 22° 30'), that 

 of September and January differs nearly 8 millimetres (0*315 

 inch), whilst at Bagota (north latitude 4° 30'), nearer the equa- 

 tor, the monthly mean does not swerve 1| millimetre" (0*059 

 inch). And in a note to the above, these means are thus given : 

 — "Rio Janeiro ; mean height in December, 337 - 02 lines (ther- 

 mometer 25*7° Ct.); in August, 340-59 lines (therm. 22-1° Ct.); 

 at the Havannah, in September, 761'23 mm (therm. 28-8° Ct.); 

 in January, 768*09 mm (therm. 21-1° Ct.). Reduced to the 

 temperature of zero (Cent.), the difference near the tropic of 

 Capricorn is 8'3 mm (0*327 inch), near the tropic of Cancer 

 7.9mm (0-311 inch)." 



But if this reasoning accounts for the violence of tropical 

 storms, it also accounts for the less violence of those of high 

 latitudes; because, as the south wind will sustain a loss of 

 force by every descent, and as the north wind in the part of 

 its course near its termination will frequently be an ascendino- 

 current, and carry with it its motion from north, thereby 

 retarding the progress of the air flowing in the opposite direc- 

 tion; the frequent alternation of these winds upon the surface 

 of the earth in high latitudes, prevents its attaining the force 



the regions of "variable winds" is, however, without doubt influenced by 

 the prevalence of particular winds, but that this is not the sole cause of 

 difference is very evident. 



