413 
of Edinburgh, Session 1876 - 77 . 
at different rates of speed. Those being most rapid where the 
arrow heads are most numerous. Let these layers also exhibit 
equal masses of air, but of different volumes, which increase in 
size from the ground upwards, accompanied also by much greater 
mobility, as shown by Tyndall in his experiments at Chamouni and 
the summit of Mount Blanc. An important point is this. The surface 
current D D' has only a horizontal source of supply from the 
direction D' from which it is fed, while the upper current A A' 
has not only a horizontal source of supply from A', but it also 
derives supply from the slower moving current B B' beneath it, 
which will be drawn upwards, or “ lifted,” in the direction of the 
small inclined arrows abed. To enable this upper current to 
supply itself in this way, while it possesses the same amount of 
mass as that beneath it, it must have a greater velocity, and 
therefore greater momentum than that of the current beneath it, 
from which it draws its supply. Each of the lower currents, as 
they approach the surface, are also supplied in the same manner, 
but in a decreasing ratio, from those beneath them, until the lowest 
layer is reached, which is that which is most retarded, not only 
owing to its proximity to a resisting surface, but also to the 
scarcity of supply, which can now only be derived from a horizontal 
source, and not from beneath, as was the case with those above it. 
A tendency in the air to accumulate aloft will now take place, by 
“ lifting,” in the direction of the small inclined arrows abed. 
Pressure is thus diminished at the surface, while it is abnormally 
increased higher up. 
The effect of this will be that the surface barometer will exhibit 
diminished or fictitious pressure, while the real weight of the 
atmosphere remains unaltered. A partial vacuum is found on the 
lee-side of a wall, over the top of which a strong wind blow's : it with- 
draws the air there to such an extent, that it causes removal to 
exceed restoration, but it does not affect the real vertical mass of 
the column overhead except to an infinitesimal degree. In this, 
as in the former case, real pressure cannot be ascertained by the 
surface barometer. It is only to be obtained from the result of an 
observation of a series of barometers placed vertically above each 
other, and not very far apart. From the results thus obtained, it 
would be found that the normal upward diminution of pressure 
