613 
of Edinburgh , Session 1874 - 75 . 
rents. This may be sufficient, insufficient, or more than sufficient. 
The supply to the uppers may differ in amount from that to the 
surface currents. The position and the distance of the source of 
supply are also important. This may he vertically or horizontally 
situated. According to Redfield and others, the horizontal extent 
of an atmospheric disturbance is often two hundred times greater 
than its vertical height. The arresting effect of such an extensive 
surface on the supply drawn over it must he great. But if supply is 
derived from a vertical source, as in the case of a descending current, 
much less retardation will take place. Hence when the supply to 
the surface current is from a horizontal source, great inclination of 
columns will take place, but when from a vertical source, there 
will he less inclination. An important difference in the mode of 
inflow of the different winds will thus exist betwixt those vertically 
and those horizontally supplied. The former will move freely in 
nearly upright columns, the latter in columns more or less inclined. 
Gradients. — A. river flows on an incline, by the amount of which 
its velocity and volume are regulated ; hut the river itself exerts a 
reactionary influence on this incline, which it will tend to pull 
down and lower, if it is not composed of rigid materials, An aerial 
gradient is not rigid, it is elastic and mobile, and being thus 
subject to the reactionary influence of the air which it draws to 
itself, it will not remain stationary, nor will its incline remain 
unaltered. Its efficiency and the amount of its slope will there- 
fore depend on the amount of facility with which air inflows to 
it. If the inflow takes place in vertical columns, little or no 
reactionary influence or lowering will be produced; but if the 
inflow is in inclined columns, which therefore produce difficult 
supply, being from a horizontal source, the tendency will be to pull 
down and lower the gradient, and thereby remove the source of 
supply to a greater distance. What thus takes place is popularly 
expressed by the phrase, that the wind blows itself out, which is in 
fact accomplished by lowering the gradient, and removing the 
source of supply to such a distance that it is almost entirely arrested 
by the extent of the resisting surface over which it is now com- 
pelled to pass. Thus a gradient represents not only a motive force, 
but also a reactionary force which is due to it. 
Curve of Outward Propagation . — There are thus two different 
