A. Guyot—Dry Zones in both Hemispheres. 165 
laws of the general circulation of the win 
it be so it is evident that modifications in that system will 
affect the position of the dry zones. e southern zone, for 
Instance, is somewhat nearer the tropic for the reason that the 
middle, or central line, of the belt of calms is on the north of 
the equator, removing the whole system of trades and anti- 
trades toward the north, an effect due, no doubt, to the pre- 
dominant extent of the land masses in the N. Hemisphere. 
Though this cause of dryness in these particular latitudes 
may be considered as by far the most efficient, secondary causes 
are not wanting which intensify locally the phenomenon, or 
give it a greater extension. It is to be noted that most of these 
phenomenon, only due to local causes, but it depends upon the 
ds. 
clear sky, with diminished density of the air, becomes strongly 
heated, keeping the air on the plateau warmer than the air at 
favors in these subtropical latitudes an accumulation of heat 
Which adds another element of dryness, and which tends to — 
merease the breadth of the dry zone. : 
The rains, often quite abundant, which fall in the portions 
of the subtropical zones situated on the east side of the great 
land masses and which have been indicated above as notable 
