182 SCIENCE IX SHORT CHAPTEBS. 



end or bottom downward, the barometer-tube is inverted, and 

 has its open mouth downward. This open mouth is either 

 dipped into a little cup of mercury or bent a little upward. 



Why does not the mercury run out of this lower open end 

 and overflow the little cup when it is inverted after being filled ? 



The answer to this question includes the whole mystery and 

 principle of the barometer. The mercury does not fall down 

 because something pushes it up and supports it with a certain 

 degree of pressure, and that something is the atmosphere which 

 extends all round the world, and presses downward and side- 

 way and upward in every direction, in fact with a force 

 equal to its weight, i.e. with a pressure equal to about 15 Ibs. 

 on every square inch. A column or perpendicular square stick 

 of air one inch thick each way, and extending from the surface 

 of the sea up to the top of the atmosphere, weighs about 15 

 Ibs. ; other columns or sticks next to it on all sides weigh the 

 same, and so on with every portion ; and all these are forever 

 squeezing down and against each other, and, being fluid, trans- 

 mit their pressure in every direction, and against the earth and 

 everything upon it, and therefore upon the mercury of the 

 barometer-tube. 



We have supposed the air to be made up of columns or 

 sticks of air one inch each way, but might have taken any other 

 size, and the weight and pressure would be proportionate. 

 Now mercury, bulk for bulk, is so much heavier than air that 

 a stick or column of this liquid metal about 30 inches high 

 weighs as much as a stick or column of air of same thickness 

 reaching from the surface of the earth to the top of the atmos- 

 phere ; therefore, the 30-inch stick of mercury balances the 

 pressure of the many miles of atmosphere, and is supported by 

 it. Thus the column of mercury may be used to counter- 

 balance the atmosphere and show us its weight ; and such a 

 column of mercury is a barometer, or " weight measure." 

 The word barometer is compounded of the two Greek word* 

 baros, weight, and metron, a measure. 



If you take a glass tube a yard long, stopped at one end and 

 open at the other, fill it with mercury, stop the open end with 

 your thumb, then invert the tube and just dip the open end in 

 a little cup of mercury, some of the mercury in the tube 

 will fall into the cup, but not all ; only 6 inches will fall, the 

 other 30 inches will remain, with an empty space between it 

 and the stopped end of the tube. When you have done this 

 you will have made a rude barometer. If you prop up the 



