93 
1913-14.] Principia Atmospherica. 
for the north-to-south current may possibly come from the surplus of the 
south-to-north current, but it cannot be along the surface. It must be remem- 
bered that, so far as our information goes, we have no reason from observa- 
tions for supposing that the relation between pressure and temperature in a 
northerly current is different from that in a southerly current, though the 
evidence is not quite conclusive, because the former has been less frequently 
the subject of investigation. The air supply ought, therefore, to be carried out 
in a similar manner in both cases. Persistence in this case, therefore, requires 
the surplus of the adjacent southerly current and the outflow from the 
northerly itself both to be delivered to the northerly current in the upper 
layers in order that the proper temperature distribution may be obtained. 
CROSS SECTION OF 9 Kl LOMETRES&K TO lOK] OF 
TWO CURRENTS S. TO N. AND N. TO S. EACH 
5‘ WIDE MAINTAINED IN LAT4 5.' 
Such a combination of circumstances may fairly be regarded as exceptional, 
and therefore the maintenance of a northerly current must be regarded as 
exceptional. 
Changes from the Steady State. 
To complete the process of maintenance of the steady current from the 
north we should have to imagine the whole of the outflow in fig. 2 towards 
the “low” from both sides conveyed to the upper part of the northerly 
current, and thus transferred from low pressure to high pressure as well as 
from low level to high level. It is possible to make out a process with the 
aid of the law of convection if the two currents are at different tempera- 
tures. In such a case the surfaces of equal pressure may be so sloped as to 
produce an apparent flow across isobars from low to high ; but we have no 
such obvious and automatic explanation to give in the case of the northerly 
current as in the case of the outflow of the southerly current. And, 
indeed, it was not intended to adduce the conditions for persistent main- 
tenance with the object of claiming that they are generally satisfied in 
practice. On the contrary, the adjustment of the outflow in the southerly 
