GENERAL SCIENCE 



tube is pressing on the mercury in the dish and the weight 

 of the air is also pressing on the mercury in the dish. 

 These two forces per unit area must be equal as the 



mercury does not flow into 

 the tube or out of it. Hence 

 the air pressure on the mer- 

 cury in the dish is equal to 

 the pressure of the mercury 

 in the tube. By weighing 

 the mercury in the tube, it 

 can be found that it exerts a 

 pressure of about 15 pounds 

 per square inch when it stands 

 at a height of 30 inches. 

 Then the air must also have 

 a pressure of about 15 pounds 

 per square inch at sea level. 

 That is, a column of air with 

 a cross-section of one square 

 inch, extending upward as far as there is air, weighs 15 

 pounds. 



If you have any doubt as to whether the open space 

 above the mercury is a perfect vacuum, just incline the 

 tube slowly and the mercury will fill the open space. 

 If there were any air or other substance in the tube, the 

 mercury could not go to the top of it when the tube is 

 inclined. Now hold the tube erect again, and the mer- 

 cury will fall to its former position. The perpendicular 

 height of the mercury is what determines its pressure and 

 not the length of the column when the tube is inclined. 

 Since mercury is 13.6 times as heavy as water, the air 

 will hold water 34 feet high in a tube when it will hold 

 mercury 30 inches high. When the mercury stands 



MEASURING THE AIR PRESSURE 

 WITH MERCURY 



