92 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The representation is, moreover, borne out by the facts which are known 
as to the distribution of temperature in the atmosphere. For the seven 
kilometres between the 1-k. level and the 8-k. level the temperature on the 
“ high ” side is “ too warm,” and therefore represents the effect of a down- 
ward flow while the pressure is maintained.* Hence it seems possible for 
the conditions for the maintenance of a south-to-north current to be realised 
in practice, though the adjustment would be delicate and might certainly be 
transient. 
Motion from North to South. 
Persistence in the reverse of the case just described, that is to say, in 
the case of a current flowing from north to south, is in one respect more 
difficult and in another more easy. 
What we have v to provide for here is not the thickening but the 
shrinkage of the current in consequence of the decrease of sin X as successive 
circles are crossed. The numerical result applies equally, but in the opposite 
sense. Thus a current of velocity V flowing from north to south requires 
that air should be fed w T ith an inflow which, if distributed over the whole 
side, would be *0175 
cos 2 X 
sin X 
IV at any level at which the wind velocity is 
V t in order to avoid fractional shrinkage of *0175 cot X per degree of 
advance. It is more difficult to see how the air could be supplied ; but the 
shrinkage of the current while the distribution of pressure which controls 
it is maintained presents little difficulty if the current in question may be 
supposed to remain an upper air-current and therefore subject only to the 
pressure-distribution appropriate to the current. To explain the persist- 
ence of a current in the lower layers would make greater demands upon 
one’s ingenuity, because the introduction of the necessary air would, as a 
rule, alter the distribution of pressure below, and limitations to prevent 
that alteration would have to be invented. Hence the maintenance of a 
current from north to south at all levels requires some artifice for the 
continuous production of the necessary pressure-distribution. The difficulty 
is further aggravated by the fact that, just as in the case of the south-to- 
north current, there is a flow-off from “ high ” to “ low ” in the surface 
layers ; but unfortunately it flows away from where it is required to make 
up the loss due to change of latitude, and consequently that loss as well as 
the loss by shrinkage has to be made good if the northerly current is to be 
maintained. 
Putting the two currents side by side as in fig. 2, we see that the supply 
* See the paper in the Journal of the Scottish Meteorological Society already referred to. 
