76 Proceedings of the Koyal Society of Edinburgh. [Ses's. 
If, however, a depression comes into existence and then dies out, the 
air which is passing while it dies out is more accelerated in its approach 
than it is retarded as it recedes, and is therefore permanently accelerated. 
Hence the process of dying out of depressions or increase of intensity of 
anticyclones is associated with a permanent acceleration of those portions 
of passing currents w T hich come temporarily under their influence. Such 
accelerated portions of a main atmospheric current, overtaking the portion 
ahead, are pressed back again, thus setting up an elastic oscillation length- 
wise in the stream and producing the phenomenon of squalliness. This 
quality is therefore not confined to the surface layers of the wind 
(although admittedly greatest there owing to surface irregularities) but 
may occur at all levels in consequence of variations of barometric pressure. 
As the permanent low-pressure zone in the region of the heat equator 
will probably be quoted as contradicting this theory of low pressure being 
due to cooling, I will discuss it now in more detail. In the tropical regions 
the air is being permanently heated from below and the rate of heating is 
so great that the vertical equilibrium must often be disturbed, giving rise 
to convection currents. The air so rising must pass entirely through the 
isentropic zone, and its inertia will carry it some distance into the over- 
lying regions of greater entropy before . it comes to rest. As it falls back 
to its proper entropic level it abstracts heat from the surrounding air 
(whose entropy is higher than its own), the conduction of heat being 
aided by eddies and mingling due to the vertical motion. Thus an effect 
is produced similar to that which would ensue from mechanical churning. 
Entropy is withdrawn from the higher layers to the lower, and the lapse 
rate is much increased. In consequence of this high lapse rate (notwith- 
standing the increased entropy in the lowest layers), the upper portion of 
the air is cooled much below the average temperature of air at the same 
level in surrounding districts. Thus the value of I is increased 
J o “ 
and the barometric pressure in the upper regions is reduced. A cyclonic 
system results, causing an expulsion of the upper cold dense air much in 
excess of the inflow of air below. Hence low pressure is established in the 
surface layer also and a permanent low-pressure belt results, in conse- 
quence of the mechanical churning set up by the disturbance of the en- 
tropic equilibrium. The July world-minimum is over the Himalaya, where 
the contours aid the churning. The Antarctic plateau (about 10,000 to 
13,000 feet) has a similar effect. 
It has been stated that the principal causes of secular radiative cooling 
or heating are probably movement in latitude, formation of extensive 
