216 
Proceedings of the Royal Society 
the atmosphere caused by the introduction of the element of 
dynamical motion is due to this accumulation of the air aloft, and 
is consequently accompanied by increase of pressure there, and by 
diminution of pressure on the surface. Let the arrow AB represent 
A B 
S S' s r 
S 7 s s 
R S 
Diag. No. 3. 
the strong upper current, and let the diminution in length of the 
arrows beneath it represent the diminution in the rate of their 
speed. As was shown in the diagram in the last paper* on this sub- 
ject, these strong upper currents obtained much of their necessary 
source of supply by “ lifting” it from the slower and more retarded 
surface currents beneath them. The direction in which this lifting 
takes place is shown by the small arrows inclined in the direction in 
which movement takes place. Accumulation is here indicated by the 
comparative closeness in the parallel position of the arrows, while the 
rarefaction and diminution of pressure on the surface, due to lifting, 
is indicated by their comparative wideness in the parallel position of 
the arrows there. It is only with a highly elastic fluid such as air, 
aided also by a certain amount of viscosity, and accompanied by 
horizontal surface retardation, that this alteration in the position of 
vertical pressure can take place. If the moving currents are non- 
elastic fluids, such as mercury, their rapidly moving upper strata 
will only cause a real diminution of pressure on the surface, but no 
increase of pressure or of accumulation aloft, because lifting cannot 
here take place. With a perfect fluid, also, no surface retardation 
will be found, and no difference in the speed of currents in so far as 
they are due to this cause. Hence here, as on a frictionless surface, 
“ lifting ” will not take place. 
When a very rapid rise of the barometer takes place, as is fre- 
quently to be found in West Indian hurricanes, it may at least so 
far be accounted for in this way ; when abnormal upward diminu- 
tion of pressure is found due to accumulation aloft, as shown in the 
diagram No. 3, and when the motive force which produces it and 
* Proc. Roy. Soc. Edin. 1876-77, p. 412. 
