VI.] THE ATMOSPHERE. 89 



its pressure upwards. Extend your hand, and you feel no 

 pressure, though it is certain that every square inch of its 

 surface bears a pressure of nearly 15 lbs., and the entire 

 hand must therefore sustain a very large total pressure ; 

 but the weight upon the upper surface is counteracted by 

 the upward pressure of the air on the under surface, the 

 two equal and opposite pressures neutralising each other. 

 Nor is there any tendency for tlie hand to be crushed 

 between these opposing pressures, for the air and other 

 fluids in the vessels and various tissues of the body press 

 equally in all directions, so that any pressure from without 

 is perfectly counterbalanced by an equal pressure from 

 within. The thinnest soap-bubble sails safely through the 

 air, though its outer surface must sustain a pressure of 

 many pounds ; the elasticity of the air within the bubble 

 causes it to press forcibly against the inner wall, and thus 

 resist the external atmospheric pressure, and effectually 

 prevent collapse. In the common toy known as "Jack 

 in the Box" a spring inside the figure presses upwards 

 against the lid when tightly shut down ; and in like manner 

 the walls of a closed vessel containing air are pressed 

 outwards by the elastic force of the confined air. If the 

 air be removed from the interior of a closed vessel, so as 

 to leave a space altogether empty or vacuous, the pressure 

 of the external atmosphere becomes at once evident, since 

 it is no longer counterbalanced by any force from within ] 

 a thin glass vessel, for example, may easily be shattered by 

 sucking the air from its interior. 



It is easy to measure the amount of this atmospheric 

 pressure by a simple experiment, first made in 1643, by an 

 Italian philosopher named Torricelli. Take a glass tube, 

 rather more than 30 inches in length, closed at one end 

 and open at the other ; fiU this tube with quicksilver, and 



