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of Edinburgh, Session 1874 - 75 . 
causes a local or partial reduction of pressure, while the real ver- 
tical pressure of the atmosphere overhead remains unaltered. The 
relation which exists betwixt pressure and the speed of the winds 
is altered as their velocity increases, in a somewhat similar way to 
that which takes place when the lee way of a ship is practically 
diminished by an increase in its head way. 
It must be observed, that lifting can only take place where 
scarcity of supply exists. The vacuum formed behind a wall over 
which the wind blows is due to the fact, that removal is there 
greater than restoration; for if supply was sufficient no such 
vacuum could exist. 
Water flowing from an orifice in the side of a cistern, which is 
only a little below its surface level, will fall directly downwards ; 
but if the level is raised much above that of the orifice, the water 
passing through it will he expelled with considerable force ; it will 
“lift” and take a form approaching to that of a horizontally 
flowing spout. The great mass of water will accumulate in the 
upper part of the curve of the spout, and will connect itself with 
the side of the cistern by a thin film of water, which will now 
take the place of the large body of water which fell vertically down- 
wards when the pressure was less. This accumulation may he 
taken to illustrate what takes place in the upper part of the atmo- 
sphere, while the thin film may represent the diminished pressure 
at the surface. 
Since scarcity of supply exists in the advancing portion of a 
progressing depression, it is there that lifting is most highly 
developed. Copiousness of supply is found in the rear, and hence 
it is there that lifting is least likely to he found. 
Lifting takes place where inequality exists in the movement of 
the various atmospheric layers, hence for this, among other reasons, 
mountain heights cannot be measured during the prevalence of 
strong winds, nor is the reduction of the barometer from consider- 
able heights to sea-level at all to he depended upon. Lifting is 
always preceded by removal of air; hut so far as removal alone is 
concerned, it is accurately represented by the barometer. The 
diminished pressure at the surface due to lifting is also correctly 
exhibited by the barometer there placed, but in such cases the 
barometer fails to exhibit the real vertical pressure due to the 
