Ill 
1913-14.] Principia Atmospherica. 
The next step in the consideration rests upon the fact that by superpos- 
ing a cyclonic depression upon the circumpolar circulation we displace a 
part of that circulation to the southward and reduce the northern part. 
Taking the case of Teisserenc de Bort’s map for January, the westerly run 
of isobars over America and Asia is about 10° to 20° of latitude lower than 
over the oceans, and these two positions of westerly circulation have to be 
connected by isobars which cross the parallels of latitude, and therefore 
have a south-to-north and a north-to-south component respectively. There- 
fore, they can only be maintained persistently under the conditions set out 
in Proposition 1. Now, it has been shown in the discussion of Proposition 1 
that permanence of a quasi-steady character might be realised in the case 
of an anticyclonic ridge having a south-to-north current on its western side, 
and vice versa, provided that momentum was being taken out of the 
westerly circulation in order to provide a slight eastward deviation from the 
isobars setting to the north. Such a case would be fairly represented by the 
deviation from circular isobars shown over the oceans on Teisserenc de Bort’s 
map for January, and hence the form of those isobars may be arrived at by 
the influence of a steady flow-off of air down the land-slope of the Arctic 
regions and the steady deviation of the wind from the direction of the 
south-west to north-west isobars on the western sides of the oceans in con- 
sequence of the momentum of the westerly circulation. 
Meanwhile, what happens to the cold air which has run off the land- 
areas ? That has to be steered about by the distribution of pressure in the 
upper air as modified by any special peculiarities of temperature in the 
lower regions, and all sorts of complications may arise from this cause. So 
far as it goes, its density tends to set up high pressure over the regions 
which it covers, and so to make a slope of pressure southward and cause 
easterly winds on its southern side. Whenever in a mass of air tempera- 
ture-fall is in the opposite direction to pressure-fall, great change in the 
horizontal distribution of pressure underneath is the result, and many of 
our local variations of pressure may fairly be attributed to the reactions 
which these cold masses of air offer to the attempt (in the end futile) on 
the part of the upper air to steer them round the pole from west to east. 
By their eastward motion these masses of cold air are always reminding us 
that if left to themselves, without the overpowering guidance of the 
pressure-distribution of the upper air, they would form a circulation round 
the pole in opposition to the circulation of the upper air, with which they 
are in perpetual conflict. 
