91 
1913-14.] 
Principia Atmospherica. 
metres per second there must be a cross wind leaving the lowest half 
cos 2 A 
kilometre of "35 — IV metres per second. 
sin A 
We have supposed the drainage to take place entirely in the lowest 
half kilometre, which represents one-twentieth of the atmosphere. The 
same result might be produced by a distributed cross-flow throughout the 
western vertical section of the moving air of *0175 C - ? S ^ P metres per second. 
We may therefore sum up the conclusion as follows: — 
In order that a current across circles of latitude from south to north 
with a breadth of l degrees of longitude may 'persist unaltered at any level, 
it is necessary that air should be drawn away from the moving air at that 
level to the extent of *0175 CQS ^ IV metres ver second. 
sin A 
The use of the surface layer, to draw off the excess of air which would 
otherwise prevent the persistence of a current across circles of latitude, is 
quite appropriate in the case of currents with a south-to-north component. 
According to the rider to Law 1, such a current certainly exists, and it only 
requires its magnitude to be adjusted in order that persistence may be secured. 
Fora current extending over 10° of longitude in lat. 45° the cross component 
CROSS SECTION OF 9 Kl LO MET RES &K.TO 10K] OF 
A S. TO N.CURRE NX 5'WIDE'MAINTAINECT IN LATHS' 
w 
DOWNWARD FLOW. OF 0175 
*TMOSP"£f?r FOR each Decker 
OF L AT n UOC CR OSS CO 
K 5* LO/VG: il 
Fig. 1. 
at the extreme west of the lowest half kilometre would have to be two 
and a half times the steady south wind above, and that hardly occurs in 
practice ; but there are a variety of ways of accounting for any discrepancy 
between the calculated and observed cross-wind in case the south-to-north 
current is actually maintained. Hence the diagram, fig. 1, representing 
the conditions for maintenance of a south wind across a section of 5° of 
longitude is not unreasonable. 
