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284
THE POPULAR SCIENCE MONTHLY.
As soon as I blow gently on the upper surface of the gravel, you see the liquid in the manometer moving. The motion of the air which I produce acts in the first instance on the surface of the gravel, propagates itself through the same to the bottom of the cylinder, enters
Fig. 2.
the lower end of the tube, rises through it and through the tubing into the manometer, where it presses on the column of liquid, and sets it in motion.
Why does the liquid move in the manometer? Because the air, after the migration just described, presses with greater weight on the
