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Proceedings of the Royal Society 
sequently fall at places which it would not otherwise have reached ; 
it will there exhibit fictitious pressure, and what takes place may 
be explained by the use of the term “ falling for sympathy with 
adjoining barometers.” 
A very considerable portion of the numerous barometric oscilla- 
tions, which so constantly take place, are an effect due to friction 
from a resisting surface. On a frictionless surface their amount 
would be greatly reduced ; no difference would then exist betwixt 
the speed of surface and upper currents; and depressions, with their 
accompanying disturbances, would then no longer possess a self- 
moving power, — they would cease to move forward, except, perhaps, 
in imbedding currents. 
Conclusions. — When a barometric depression is formed, the 
winds inflow spirally towards the centre, but they very seldom do 
so equally or uniformly. In front they do so with difficulty, owing 
to the peculiar mode of inflow which there takes place ; in the rear, 
they do so with comparative facility. In these circumstances, the 
low central barometer cannot remain stationary. It will move 
forward in that direction in which supply is most scarce, and by so 
doing, it will be enabled to procure the necessary amount of supply, 
which it could not have received if it remained stationary. It is 
in this way that progress takes place. A depression thus possesses 
within itself a self-moving power. When a barometer begins to 
fall rapidly, the fall may extend itself uniformly all round over the 
surrounding area. Such uniformity of extension, however, does 
not usually take place, except when a depression remains stationary. 
It generally assumes some particular direction , which is that indi- 
cated by progress. This is due to the difference betwixt dynamical 
and statical pressure in front and rear, or, perhaps more frequently, 
to the difference betwixt the amount of dynamical pressure to be 
found in these positions. 
When the atmosphere is in a state of perfect rest, the barometer 
then indicates real vertical pressure ; but when it is in motion, and 
the surface currents are retarded by friction, while the dynamical 
element of motion is introduced in a comparatively larger propor- 
tion into the uppers, the process of lifting takes place, by which 
surface pressure, as indicated by a barometer there placed, is 
reduced, while the real vertical pressure of the column of air aloft 
