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is to be regarded as being an effect. In another point of view, it 
may be here mentioned, that if a very swift wind blows over a 
convex surface, pressure will then be diminished, but if the con- 
vexity is that of the earth’s surface, and the velocity that of actual 
winds, the effect will be far too small to be sensible. 
These rapid upper currents and their effects, to which so much 
importance has been attached, do not consist merely of light rarefied 
upper aerial strata which can be possessed of little momentum. 
The mode in which they are constructed may be thus described. 
Their velocity begins to increase at only a few feet above the surface, 
as has been shown by observations ; this increase goes on often to 
a great height in the atmosphere, much of it, however, is to be 
found in the more weighty and condensed mass of the air not much 
above the surface, where it must be accompanied by a powerful force 
due to its momentum. The weight of a cubic mile of air amounts 
to several millions of tons ; where many such are in motion, their 
weight and their accompanying momentum will be sufficiently pro- 
digious to produce the effects which have been pointed out. 
From experiments made by Halley and Hawksbee, they came to 
the conclusion that horizontal movement took off vertical pressure, 
while Professor Leslie experimented to refute this.* A well-known 
experiment will illustrate this. If a current of water flows down an 
incline in the direction of the arrow AB, instead of falling down 
through the orifice at C, it passes over it and draws up or lifts 
through the small tube CD the water contained in the vessel 
GHKL. This is carried out at the expense of the momentum 
of the greater current, in which, in this way, an additional 
amount of water is accumulated. The gravity of the current as 
* See “Trans. Koy. Soc. Edin.” vol. xx. p. 377. 
