VIBRATIONS OF THE AIR.—­If a person stand beneath a railway girder-bridge with an open umbrella over his head, when a train is passing, the vibration of the air will be distinctly felt in the hand which grasps the umbrella, because the outspread surface collects and concentrates the waves into the focus of the handle.

THE EARTH’S CENTER.—­All bodies weigh less the further removed they are from the center of the earth.  A block of stone weighing 700 pounds upon the sea-shore, will weigh only 699 pounds if carried up a mountain three miles high.  A pendulum oscillates more quickly at the poles than at the equator, because the earth is flatter by twenty-six miles at the poles—­that is, the “bob” of the pendulum is that much nearer the earth’s center, and therefore heavier, and so swings more quickly.