132 Baron Leopold Von Buch on the Subtropical Zone. 



flowing perpendicularly over one another, flow alongside each 

 other. The shape of the land and of the sea, different tempera- 

 tures of the soil, different velocities of the wind, and frequently 

 their crossing one another, change these polar and equatorial 

 streams in very short periods, and the barometer continues in a 

 constant state of agitation. The law of these vibrations will no 

 longer be general, but belong to every individual part of the 

 surface of the earth in which they occur. The hmits of the 

 subtropical zone, on the side towards the equator, will be deter- 

 mined by the tropical rains, which is, at the same tiniCj the far- 

 thest limit which the upper equatorial current flowing towards 

 the pole, in the winter, touches the surface of the earth. The 

 boundary of the zone, on the side turned from the equator, is 

 determined by the disappearance of the law of the regular de- 

 crease and increase of the mean barometi'ical heights. In sum- 

 mer, they would be where the upper equatorial current some- 

 times descends down, but, in winter, dispute the place with the 

 north-east wind to the surface, and flow with it, for the most part, 

 alongside each other, but in opposite directions. These limits 

 may be determined, in the northern hemisphere, in the following 

 manner. The southern limit will fall somewhat to the north of 

 the Cape de Verd Islands, about the 20° of latitude ; in the in- 

 terior of Africa somewhat more to the south ; for Deiiham ex- 

 pressly remarks, that the tropical rains begin on the west coast, 

 in the 16° of latitude. In Nubia, also, according to Ehrenberg, 

 they first begin in north latitude 16°. The northern limit will 

 be placed over Cairo, yet not directly by Algiers ; therefore in 

 the 32° of north latitude. In the southern hemisphere, this 

 boundary extends nearer to the pole, as is sufficiently proved 

 by observations made at the Cape, in south latitude 33°. But 

 it does not extend much farther, for the few observations made 

 at Buenos Ayres appear to assign that place a situation beyond 

 the boundary of the effect of the law of the medium barometri- 

 cal heights. That Santa Fe' de Bogota, although in the north- 

 ern hemisphere, ye*, follows in the mean barometrical heights 

 the curve of southern places, is a beautiful confirmation of the 

 encroachment of southern climatic relations beyond the equator. 

 The boundary of the southern trade wind passes, in summer, 

 from 7° to 8° beyond the equator ; and in the estuary of the 



