616 Proceedings of the Boyal Society 
and where, consequently, it is most difficult to lift, and where the 
drag is greatest. 
In a somewhat similar manner the low central barometer, having 
by means of the peculiar mode of inflow of winds in that segment 
caused supply there to be scarce, will itself move in that direction , 
to obtain the requisite supply, which it could not procure if it 
remained stationary: in so doing, it opens out the depression in 
front, and is enabled to move forward, provided it is sufficiently 
supplied in the rear. If high pressure or steep gradients existed 
in front of one of the segments, progress could not there take 
place, since supply being there abundant, no lowering of the 
gradient could take place, nor could it shallow itself out in that 
direction. 
Lifting . — From the greater mobility of the atmosphere in the 
upper regions, it there moves faster, and hence the air is more easily 
removed than it is near the surface. The atmosphere may thus 
be conceived to be divided into a number of spherical concentric 
layers, each possessing a different rate of speed, slipping or sliding 
over those underneath with an increasing amount of friction, as 
their position becomes lower. The upper layers possess two sources 
of supply — one from a horizontal source, the other from the layers 
underneath, while the surface layers possess only a horizontal 
source of supply. The facility with which the uppers are thus 
supplied, tends in the first instance to increase their speed, but 
when this has taken place to a certain extent, the source of 
supply will diminish in amount. This is accompanied by a 
lowering of the gradient, the effect of which is to remove the 
source of supply to a greater distance, and increase the diminu- 
tion, until a point is at last reached in which it is almost entirely 
arrested. When this begins to take place, the uppers will tend to 
lift and become detached as it were from the surface, thus causing 
a partial vacuum near the surface. 
Lifting may be illustrated by what takes place on the lee side of 
a house or wall, over which a strong wind blows, a partial vacuum 
is here formed. The friction which retards the air when flowing 
over an extensive horizontal surface, may be represented by a 
series of such obstructions which enable the air to be more easily 
carried off and removed than it can be restored. This removal 
