215 
of Edinburgh, Session 1878 - 79 . 
it passes over the orifice at C is undiminished ; notwithstanding 
this, it does not here descend. Why it does not do so may 
he thus explained. Its gravity, which operates in a downward 
vertical direction AC, is here altered by its horizontal velocity, 
Diag. No. 2. 
AB in the direction of the component AD. Attraction to the 
earth’s centre remains undiminished ; but if in this way it is distri- 
buted over a greater extent of surface, on any point of it, it may 
then practically be regarded as being really diminished. The weight 
of the current passing over the orifice at C may therefore be regarded 
as operating in an altered direction. With an incalculable amount 
of velocity, gravity may be said totally to disappear. The distribu- 
tion of pressure may be exemplified by a cannon ball which, if it 
falls directly downwards from the mouth of the gun, will then show 
its full weight when it comes in contact with the surface of the 
ground, but if it moves rapidly in a horizontal direction its weight 
may now be regarded as being distributed over an extent of surface, 
on any point of which it is there diminished practically. When 
skating slowly over thin ice, it gives way, but by passing rapidly 
over it, this may be done in safety. The explanation of this is, that 
the ice has not here time to give way ; this is no doubt so far correct; 
but it must also be observed, that the weight of the skater, if at rest, 
is to be found in a vertical direction and is then undiminished, but 
when he moves it is then distributed over the surface of the ice and 
is in this way lessened. The greater the amount of velocity, the 
greater will be the practical diminution of pressure over a corre- 
sponding extent of surface. 
Let a current of air move rapidly along the incline AB in the 
mode above shown. Let the vessel GHKL, which held water, now 
only contain air, air will in the same way be drawn up, lifted , 
and accumulated aloft , in the rapid motive current AB. If the 
vessel is enclosed on the top, the tube by which the air is drawn up 
from it will produce within it rarefaction and diminution of pressure, 
and only a slight accumulation aloft, as its source of supply is now 
entirely restricted. Upward abnormal diminution of pressure in 
voji. x, 2 a 
