Baron Leopold Von Buch on the Subtropical Zone. 131 

 A law distinctly follows from these observations, according to 

 which the mean barometric heights, for single months, reo-ular- 

 ly decrease from winter to summer, and this the more the far- 

 ther they are distant from the equator. Irregularities, which 

 appear in these statements, will probably disappear by a mean 

 of many years. When we consider that this law is revealed in 

 Louisiana, Cairo, Benares, and in Macao, we cannot doubt that 

 it will also be observed in Owaihi, and hence be general over 

 the earth. Rio Janeiro and the Cape-town are already so far 

 removed in longitude, and even in meteorological situation, from 

 each other, that we may expect a similar character for the south- 

 ern hemisphere. 



But this phenomenon has determinate limits. It disappears, 

 not gradually, but in a sharp transition ; and in the apparent 

 inequalities, towards the pole, of the mean barometric heights, 

 there is revealed the temperate zo?ie. In Palermo, Cadiz, 

 Mafra, we find no longer the deepest mean heights in the 

 summer, and still less in places having a higher geographi- 

 cal latitude. As it appears to be proved, that the different 

 barometric heights and their irregularities depend on the nature 

 of the prevailing winds, it is probable, also, that the regular 

 course of decreasing mean barometric heights also depends on 

 changes of the wind ; and this is proved by phenomena in India. 

 For there it is that the rainy south-west monsoons cause the 

 sinking of the mercury, and this in proportion as they touch 

 from above downwards the surface of the earth. The north- 

 east wind raises, with equal uniformity, the mercury in the ba- 

 rometer. It is also observed, where the law of the barome- 

 tric heights no longer appears, that, in summer, the trade, or 

 the regular north-east or north wind, is sometimes overcome by 

 the south or south-west wind. But these are the upper equa- 

 torial currents, which in high latitudes come from above. As 

 they ascend everywhere in the torrid zone, and flow towards 

 the poles, they will be forced, the farther they come, from great 

 circles of the earth's surface, into circles of smaller diameter, and 

 so the farther they go into smaller and narrower limits. They 

 will therefore increase in height, as also in velocity. Lastly, 

 they will force tlie nortli-east wind to yield ; and, in place of 



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