of Edinburgh, Session 1878-79. 
217 
holds it up in this position has come to a termination, a vertical 
rapid descent of the accumulation will of course take place to over- 
come the diminution of pressure on the surface, and in this way it 
will at least so far approach the form of upward normal diminution 
of pressure. A rapid fall of the barometer on the surface will also 
in a similar but in a reverse way take place, when accumulation 
begins to take place, with diminution of pressure on the surface. 
Upward normal diminution of pressure, which is here made use of 
as an exemplification of what takes place, is in all probability seldom 
or never to be found, as the atmosphere is rarely, if ever, in a state of 
perfect rest, nor is there any uniformity in the effects of heat and 
vapour. It is only in a mechanical point of view that it can assist 
in explanations. 
When the wind, represented by the arrow AB, blows over the top of 
a wall CD, it will lift the air and diminish pressure on the lee side at 
A 
c 
0 
:d 
Diag. No. 4. 
the point 0, though only to a small amount, while the weight of the 
atmosphere may be said to be unaltered. This diminution of pressure 
is caused by the wall, which retards the velocity of the wind. Friction 
on the surface of the earth may be exemplified by a long series of 
such walls, which will produce diminution of pressure immediately 
above them. It is only on a very extensive surface of, say several 
hundred miles in length, with a corresponding width, that strong 
upper currents can produce lifting and diminution of pressure to its 
full extent ; under such circumstances, the comparative height of 
these upper currents will not be great as compared with their 
length. Redfield has pointed out that depressions have often a 
[ height of only P ar t °t their width, while in other instances 
their height may be comparatively great in reference to their width, 
in which case little or no diminution of pressure will be found. 
The effect of such an extensive resisting surface is to produce re- 
tardation of the surface currents, along with an imperfect amount 
