162 A. Guyot—Dry Zones in both Hemispheres. 
abundant summer rains of the tropical climate, and on the 
—in my Physical Geography, page 90—that these dry, parched 
tracts form two belts around the globe; two dry Zones, on both 
sides of the tropics, containing most of the so-called deserts of 
the world, in which the regime of the rains, as well as their. 
quantity, differs from that of the neighboring climatic regions. 
As this fact, however, does not seem to have been fully recog- 
nized, perhaps for the want of a sufficient development, I beg 
leave to again call the attention of the Academy to it, and to 
indicate in a few words, what I believe to be the cause to which 
this remarkable phenomenon may be referred. 
The Northern zone of dry lands extends in width from 
about 24° to 82° N. lat. (See Loomis’s map of rains, in this 
Journal, vol. xxv, January, 1888). In the New World it be- 
gins at the west with the peninsula of Lower California ; thence 
passing through Arizona, New Mexico and Western Texas. 
In all these lands, extending for nearly a thousand miles from 
west to east, the annual fall of rain remains below ten inches, 
and goes down to two and three inches, while in some years the 
rain fails entirely. Farther east, in the same latitudes, local 
causes, to be mentioned hereafter, give abundant rains to the 
valley of the Mississippi and Florida. 
n the Old World the dry zone oceupies the very center of 
the Great Sahara, where the absence of rain is nearly complete 
on a length of 3,200 miles, and is considerably increased in 
width. Thence it crosses the central part of Arabia on 4 
line of 1300 miles, passes through the dry plateau of Hastern 
Persia and Beloochistan, and reaches, beyond the Indus, the 
desert of Thurr, after a course of another one thousand miles, 
making together a tract of 5,500 miles of dry lands. 
Farther east, as in the New World, local causes bring, in the 
same latitudes, abundant rains which mask the influence of the 
general cause of dryness, as it will presently be shown. 
In the Southern hemisphere the dry zone is strongly marked 
on the west slope of the Andes. It is well known that ail the 
coast of Peru from Punta Parina 6° 8. L.to N. Chili 30° S. L. 
is a rainless region; but it would be a mistake to believe that 
the atmosphere is wanting in moisture in all that extensive 
district. All along the Peruvian coast, though owing to speci 
circumstances, no condensation takes place near the seashore 
sufficient to produce rain, the thick winter fog, called the 
