218 Proceedings of the Poyal Society 
of supply. If these are not found, “lifting” will not take place. 
The importance of the difference in the comparative height and 
length of the currents, and of the depressions, in a mechanical point 
of view, was taken advantage of in the last paper * to sho w how 
depressions opened out in front, and hv so doing moved forward. 
An exemplification of the effect of an extensive surface which 
produces low pressure may be found in the southern hemisphere 
below lat. 40°, where what are called the “ roaring forties ” are to be 
found, in often interminable gales from a westerly direction, and 
with little intermission, and extend over a vast portion of the globe. 
The Bev. S. J. Perry, in his voyage there, found the average height 
of the barometer there to be in November, 29*658° ; in December, 
29*462°; in January, 29.406°; February, 29.610°. This may there- 
fore be accounted for by removal and lifting. In other countries, 
where calms and light winds prevail, — where strong winds blow 
only for a short time over a small extent of surface, as in the 
Mediterranean, no such low pressure is to be found. In explana- 
tions attached to the barometer, to show the effects of its rise and 
fall, calms are predicted when the mercury is high, as a general 
rule. Local exemplifications of lifting from the bottom of a valley 
are to be found in the Pohn in Switzerland, where the wind 
passes rapidly over their summits, and are also to be found on the 
lee side of precipices. 
In the diagram No. 1, where the current of air AB lifts up the air 
from the vessel GHKL, which is now enclosed on the top, and has 
now its source of supply arrested, it is then that rarefaction and 
diminution of pressure takes place to the full extent. This may 
also illustrate the effect of an extensive resisting surface in arresting 
the source of supply to its surface currents and causing their retarda- 
tion. In a former paper f it was stated that, in weather charts, the 
constant rise and fall of the barometer, which is there reported, is to 
a large extent simply due to the passage of air over a resisting 
surface ; on a frictionless surface these mechanical effects would be 
entirely removed. 
There is an important difference in the mode in which Equatorial 
as compared with Polar winds, inflow to a low centre. Let these 
* Proc. Roy. Soc. Edin. 1877-78, p. 572. 
t Proc. Roy. Soc. Edin. 1876-77, p. 414. 
