572 Proceedings of the Royal Society 
similar facility, but, on the contrary, much friction must now be 
called into play, owing to the extensive resisting surface over which 
the air-currents are drawn.* Since friction is much greater near 
the earth’s surface than it is aloft, it follows that such storms are 
characterised by greatly retarded surface currents, and rapid upper 
currents. Looking at the mass of the atmosphere on the outskirts 
and outside the area of the storm as the source whence the incurring 
air-currents draw their supply, it is obvious that as regards the true 
cyclone the source of supply is easy and copious, whereas in the 
case of our widespread European storms, supply is comparatively 
scarce, and therefore defective. In truth, as the lower and denser 
air-currents are, owing to the enormous extent of surface they cover, 
much retarded by friction, the main source of supply for the inflow- 
ing currents is chiefly to be found in the rarer, more mobile, and 
more rapid upper currents which flow comparatively free and unim- 
peded. This essential difference in the mode of inflow of the air- 
currents of these two types of storms is considered to be due to 
differences in the amount of friction, accompanied also by a great 
difference in the introduction of the important element of Time. 
The difference in velocity between the surface and upper currents 
is often very great. Thus Glaisher has shown from his balloon 
ascents that the upper currents are sometimes five or six times more 
rapid than the surface currents, while from observations made at 
the top and base of Mount Washington, it was shown that on one 
occasion the wind on the summit was blowing with a velocity of 
ninety miles an hour at the time that a calm prevailed at the base 
of the mountain. 
The following illustration will show what is meant in this paper 
by the expression opening out all round. Let a barometric de- 
pression be formed several hundred miles in diameter, and let the 
stratum of air resting on the surface be calm, whilst aloft upper 
currents flow in upon the centre at the rate of ninety miles an hour. 
In this case the central inflow will be carried out entirely by the 
upper currents, and consequently there will result from this mode of 
inflow an outward extension or an opening outwards of the area of 
diminished pressure. 
In fig. 2, A B C may be supposed to represent the vertical section 
* “ Proceedings of R.S.E.” vol. viii. p. 613 and p. 614. 
