300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
its course. Instead of rising in the torrid zone and turning back 
upon itself to Antarctic regions, the southeast trade rushes across the 
equator, skirts the coasts of Africa, Arabia, India, Siam, and China, 
whirls around the great desert plateau of Tibet, producing the area 
of low pressure that is central over that region in the hot months, 
and finally is lost in Kamchatka. Of course this transfer of a great 
mass of air from the southern to the northern hemisphere during our 
summer must be followed eventually by the return of an equivalent 
mass to the southern hemisphere; but we have not yet discovered 
how, or when, or where that return is effected. Therein hes the 
secret of much of our so-called periodic or quasi-periodic and secular 
weather changes which depend on. the internal mechanism of our 
atmosphere, not on solar or cosmic influences. 
Finally, we now go one step further and note the fact that we may 
divide the surface of our globe into two hemispheres, known as the 
continental and the oceanic. The former has its pole on the Green- 
wich meridian at about 30° north, including nearly all of Europe, 
Asia, Africa, the Atlantic Ocean, and both the Americas, being 
about three-fourths land. The other has its center about 40° south, 
includes the greater part of the Pacific, Indian, and Antarctic regions, 
and is four-fifths oceanic. The sun’s heat pours upon the conti- 
nental hemisphere with especial fervency in May, June, and July, 
and upon the oceanic hemisphere in November, December, and Jan- 
uary. The circulation of the air, both horizontal and vertical, the 
distribution of temperature, moisture and pressure, the resulting 
winds and rains over the continental hemisphere in its summer have 
but slight analogy with the corresponding phenomena over the 
oceanic hemisphere in its summer, because of the differences in the 
action of insolation upon land, water, and snow or ice. We are 
no longer justified in treating the whole atmosphere as though it were 
resting upon a globe of uniform surface and subject to shght per- 
turbations by reason of ocean currents and small continents. We 
have to consider the insolation of the continental hemisphere and 
that of the oceanic hemisphere as two disturbing forces of equal 
magnitude, acting on the air above these in such a way as to cause 
these halves of the earth’s atmosphere to react on each other in a 
series of movements or perturbations most delightful to contemplate 
and most inspiring to the mathematical expert, who quickly acquires 
a grim determination to solve the problems that are presented. This 
interaction of the continental and oceanic hemispheres is responsible 
for the fact that what happens in India in its summer by reason of 
the special character of its monsoon is not only related to what hap- 
pens in Africa and Siam, but even to what happens in Australia and 
America. A most interesting evidence of the recognition of this 
principle will be found in the fact that Mr. Gilbert T, Walker, the 
