Julius Sumner Miller: Lesson 10 - Atmospheric Pressure

Put out your open hand - grab hold of a handful of air - there is NOTHING there! Or so it seems! But there is - there's a powerful lot of STUFF there -an enormous array of it! The ATMOSPHERE is a massive thing. The pressure of the air is about 15 pounds per square inch at sea level. On every square inch of everything there is a load of 15 pounds - very nearly. The average human being bears a load of some TWENTY TONS! The whole blanket of atmosphere which envelops the Earth weighs some 5000 million million tons! Fantastic! We show an array of enchanting DEMONSTRATIONS on THE PUSH OF THE AIR.
A - We boil water in a tin can. We drive out all the air. We now stopper up the can. The water vapor in the can condenses - that is - it goes back into the liquid state. The pressure in the can is reduced. The atmosphere squeezes the can! The PUSH of the air is terrific.
B - We do the same thing with another can but in this case we evacuate
the can - we take out SOME of the air - with a vacuum pump. Again the great push of the air squeezes the can.
C - A funnel has its open end covered with a stout rubber sheet. We take out some of the air. The atmosphere PUSHES the sheet in - the more air we take out the more it pushes in - and suddenly BANG - the sheet is burst apart by the PUSH of the air.
D - "Suction" cups! This is bad language! There is NO SUCTION! We
squeeze the "suction" cups together; we drive out the air between them. Then what? The atmosphere OUTSIDE pushes them together! And very strongly.
E - We do the classic experiment of Otto von Guericke with the Magdeburg Hemispheres. In the original demonstration in the Public Square SIXTEEN HORSES pulled the hemispheres apart. But only eight were really necessary. See why?
F - On a sheet of newspaper about 20" by 30" - that is - on an area of 600 square inches - there rests a load of atmosphere of some 9000 pounds. Fantastic! Now we wish to put this enormous load - this massive MASS - into motion by a short-lived impulsive blow. Remember the Sack of Bricks in Inertia? Newton said: "A body at rest wishes to remain at rest". And 9000 pounds - over FOUR TONS - has just too much inertia to be put into motion suddenly. So BANG! The board is broken because the great load of air does not wish to move.
When the air is quiet the PUSH of the air is something to think about. Imagine what happens when this massive air is on the move - as in a hurricane or in a tornado. Cities are destroyed!