Air Has Weight — The Upside-Down Glass and Torricelli's Barometer

We live at the bottom of kilometres of air, and that air has weight, pressing on everything. In 1643 Galileo's student Evangelista Torricelli proved it with a tube of mercury. You will prove it with a glass of water and a piece of card.

Fill a glass right to the brim with water. Lay a stiff piece of card flat over the top so it seals the whole rim. Holding the card in place, turn the glass completely upside down over a sink (the first tries can be wet!). Now let go of the card.

The card stays put and the water does not fall out. The weight of the whole atmosphere pushing UP on the card is far greater than the weight of the little bit of water pushing DOWN, so the water is held in. Try it with more and less water, and with different-sized glasses, to convince yourself it is the air doing the work, not the card.

What the trick teaches. (1) Air pressure at sea level is about 101 kilopascals, the same as a 10-metre column of water or a 760-millimetre column of mercury — that is why Torricelli's mercury tube stands 76 cm tall. (2) His tube also made the first sustained VACUUM in the empty space above the mercury, overturning the ancient belief that 'nature abhors a vacuum'. (3) Because the mercury height rises and falls with the weather, the barometer became the first weather instrument: falling pressure warns of storms, rising pressure brings fair skies. (4) The same idea gives us the altimeters that keep aircraft safe (pressure drops with height) and explains why a suction pump can never lift water from a well deeper than about ten metres.