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Proceedings of the Royal Society of Edinburgh. [Sess. 
current to the conditions of persistence must be fortuitous and unlikely to 
be maintained for long ; the adjustment of conditions for the maintenance 
of a northerly current is even more fortuitous. The reason for setting out 
the conditions of maintenance is rather to show that natural conditions of 
atmospheric currents are not, as a rule, those of persistence but of change. 
If the conditions of persistence which have been set out are not realised, 
the currents will change, and by Law 1 changes in currents imply changes 
in the distribution of pressure. Consequently, an atmospheric system 
which includes northerly or southerly currents has within itself elements 
and causes of change in the distribution of pressure. It is therefore 
unnecessary to attribute all changes to outside causes. It is preferable to 
consider the causes of the changes which are inherent in cases in which we 
cannot suppose the conditions of maintenance satisfied, and to regard 
external causes of change which are known to exist as supplementary. 
It follows that we have not to regard a quiescent atmosphere all over 
the globe as the starting-point of our explanation of the present condition, 
but we have rather to regard the circumstances of transition from one 
set of conditions to another. 
We may add some notes upon practical cases. 
Persistent Southerly Current. 
The maintenance of a southerly current has been shown to be a question 
of adjustment of velocities, and a southerly current lends itself comparatively 
easily to persistence. Examples of a persistent southerly current across 
the parallels of Northern Europe furnish a well-recognised type of weather 
that seems to resist the incursions of cyclones from the west. A southerly 
current often extends throughout the vertical section of the atmosphere, as 
might be expected from the automatic thickening described above. 
Persistent Northerly Current. 
On the other hand, a northerly current requires constant reinforcement, 
and yet a northerly current, persistent for days over the North-Eastern 
Atlantic, is by no means unknown. It is possible that the necessary air 
in this case may be supplied by the gravitational flow of cold air off 
Greenland or Northern Siberia, which must contribute a large amount of 
air to the surface layers above the North-Eastern Atlantic. 
Replacement of a North-Easterly Current by a South-Westerly Current. 
An example of the disturbance of persistence frequently occurs in the 
case of a north-easterly current with a south-westerly current above it, a 
