65 
1918-19.] The Origin of Anticyclones and Depressions. 
Above region of increased entropy, no change in definite portions of air, 
but pressure, temperature (except in the stratosphere), and entropy 
at any given level are altered, as shown in fig. 2, in consequence of 
vertical displacement of the air. 
This probably represents an occurrence more frequent in natural condi- 
tions than that shown in fig. 1. 
In virtue of these pressure changes, if the surrounding air was originally 
Air which has gained 
Heat. 
Air Unchanged 
but Elevated. 
Stratosphere. 
P r e s s u r 
e I n c r e a 
s e d. 
Condi- 
tions 
Temperature I 
n c r e a s e d. 
Unchanged. 
Un- 
changed. 
Density Decreased. 
Density Inc 
r e a s e d. 
Entropy Increased. 
Entropy Dec 
reased. 
BU 
= a H = 
= b 
Air which has lost 
Heat. 
Air Unchanged 
but Lowered. 
Stratosphere. 
Pressure 
Decreased. 
Condi- 
tions 
Temperature 
Decreased. 
Unchanged. 
U n- 
changed. 
Density Increased. 
Density Dec 
reased. 
Entropy Decreased. 
Entropy Inc 
reased. 
Fig. 2. 
at rest or in uniform motion, an anticyclonic circulation will arise over A, 
and a cyclonic circulation over C, at heights greater than H — x. For the 
pressure differences in horizontal planes will tend to set the air in motion 
outwards from A and inwards towards C. But the air over A shares in the 
earth’s rotation. It has, therefore, an angular momentum about an axis 
parallel to the earth’s polar axis and passing through its own centre of mass. 
In the process of expansion this angular momentum is unaltered, and therefore 
the angular velocity is diminished (since the moment of inertia increases) 
and will become less than that of the underlying terrestrial surface. To an 
VOL. xxxix. 5 
