A vacuum is a space devoid of matter. The term is generally applied to those instances in which air is drawn from within an air-tight vessel.
650. Is it possible to form a perfect vacuum?
It is probably impossible to do so, even with the most powerful instruments—some portion of air would remain, but in so thin a form that it would be imperceptible.
651. Why does the depression of a pump-handle cause the water to flow?
Because the putting down of the handle lifts up the piston with its valve closed, thereby tending to produce a vacuum; but the pressure of the air upon the water not contained in the pump, forces more water up into the part where a vacuum would otherwise be formed. Then, when the handle is raised, and the piston forced downwards, the valve opens, and the water rushes through.
There is a second valve, below the piston, which closes with the downward movement, to prevent the water from rushing back again.
"The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit."—John ii., iii.