Mr. W. Brown on the Storms of Tropical Latitudes, 277 



tion may also be deduced from observation, being shown by 

 its effect on the mean height of the barometer, as already ob- 

 served in a former paper, though it is probable that amongst 

 the causes there given as combining to produce this effect, 

 due prominence was not given to this one ; for we find that 

 the mean height of the barometer at latitude 32° is one-tenth 

 of an inch above that at latitude 22° ; in opposition to the 

 effect of the centrifugal force of the earth's rotation. Now 

 as the greater part of this distance is within the zone of the 

 trade-winds, the principal cause must be the excess of the in- 

 flux of the upper current; the lower one being retarded by 

 friction, to which the former is not exposed. But we have 

 another instance more striking than this. In some compara- 

 tive observations of the height of the barometer during dif- 

 ferent seasons, inserted by W. C. Redfield in the American 

 Journal of Science, vol. xxxviii. p. 267» it appears that the 

 mean height of the barometer at Canton is 30*246 inches in 

 winter, and 29*974- inches in summer : now Canton is situated 

 in about latitude 23°, thus much nearer the northern verge 

 of the monsoons than the southern : now the north-east mon- 

 soon (or the trade-wind) blows in winter, and the south-west 

 monsoon in summer ; thus, as the upper currents are of course 

 the opposite of these, Canton is near the influx of the upper 

 current in winter and near its egress in summer, and hence 

 the barometer stands 0*272 inch higher in the former season 

 than in the latter* . 



* The influence of the change in the direction of the currents is even 

 greater than this, for omitting the months during which the monsoons 

 change, the difference of pressure is increased to about one-third of an 

 inch ; the amount which it is necessary to reduce the elasticity of the upper 

 current in order to counteract the effect of friction upon the lower. 



In strict accordance with this variation are those of high latitudes, which 

 are the opposite of those of the monsoons; for it appears from the obser- 

 vations just quoted, that at Newfoundland, latitude 49°, the summer pressure 

 is 0145 inch greater than the winter. The effect of friction upon the lower 

 current, and that of the opposition given to it by the falling of the upper 

 one being the same, — to cause in one part an accumulation of air and in 

 another a deficiency, the accumulation will be in polar regions, and the 

 deficiency in high latitudes ; and as these currents are of so much greater 

 force in winter than in summer, their influence will of course be greatest in 

 the former season; hence this deficiency of pressure is greatest in high lati- 

 tude during winter, and the greater pressure of those near the pole ought 

 to be at the same time increased. 



The apparent exception to this in the pressure at New York, which ac- 

 cording to Redfield is 0*044 inch higher in winter than in summer, is pro- 

 bably due to its vicinity to the northern verge of the region of the trade 

 winds, and the position of the Gulf of Mexico with regard to the continent 

 of America. The mean pressure in this country for the season is thus 

 given by L. Howard :— summer, 29*883 ; autumn, 29833 ; spring, 29*800 ; 

 winter, 29*778 (Climate of London, vol. i. p. 210.) The mean pressure in 



