HYPOTHESIS OF CAUSE OF GLACIAL PERIODS 701 
the magnetic pole may prove to be another expression of the 
same correlation, made indirectly through the agency of electric 
and magnetic dynamics springing from atmospheric circulation. 
One-stded location of ‘‘ lows.’’—As already noted, the two areas 
of permanent low barometric pressure are located on the Ameri- 
can side of the globe and have their centers, according to Hann 
and Buchan, less than 140° Long. (5 of the total 360°) apart. 
They are notably elongated in a general east-westerly direction, 
the north Atlantic area being especially extended easterly and 
northeasterly, and somewhat curved and reniform. If the iso- 
baric line of 29.95 inches, which represents the average pressure 
for the globe, be taken as defining the low areas, their borders 
are only about 4o° Long. apart on the American side, while 
they are 115° Long. apart on the Asiatic side in about the same 
latitude. In other words, the distance between the borders of 
the low areas on the American side is about one third of the 
distance on the Asiatic side. The distance between the borders 
of the ‘‘lows” is only about one half their own longitudinal 
diameters. The tract between the borders of the “lows” on the 
American side being thus relatively narrow, it might naturally be 
anticipated that the currents within it would be much influenced 
by those of the adjacent depressions, and this seems to be ina 
large measure realized, for the winds on the northern American 
plains east of the Rocky Mountains flow in the main concentric 
to the north Atlantic depression. On Buchan’s map 52, which 
has a polar projection, the isobaric line of 30 inches describes. 
nearly a circle about a point not far from the center of Green- 
land’s ice-field. (See sketch map 2.) In a rude way the pre. 
vailing winds within this circle and for some short distance 
without it whirl about the Greenland center with inward tend- 
ency. The influence of the north Pacific depression does not 
appear to be appreciably felt east of the mountains. 
Relation of moving cyclones.—Ilt is interesting to note in this 
connection that many of the moving cyclones that traverse the 
mid-latitudes of our continent seem to take their origin in the 
tract between these two permanent cyclonic areas, or in the 
