60 ELEMENTARY SCIENCE 



thirty inches. Since mercury weighs a half-pound to the 

 cubic inch, it follows that the standard atmospheric pres- 

 sure amounts to fifteen pounds to the square inch. Simi- 

 larly, in barometric readings, thirty inches is taken for the 

 standard. Pressure which the barometer records 

 1 f* by standing at over thirty inches is said to be 

 high; pressures recorded at less than thirty 

 inches are said to be low. You already know 

 that a barometer is used to foretell weather, and 

 weather is largely due to states of atmospheric 

 pressure. If the conditions of atmospheric pres- 

 sure over a large area are known, the weather 

 about to follow can be foretold with a good deal 

 of certainty. 



The standard of atmospheric pressure (fifteen 

 pounds to the square inch) is also used as a 

 standard in measuring other kinds of pressures; 

 it is referred to as a pressure of one atmosphere. 

 Thus the pressure of water may be measured in 

 atmospheres. Water at thirty feet below its 

 surface has a pressure of one atmosphere, etc. 

 Evidently this is a matter of importance in 

 under-water constructions, as in building of sub- 

 marine boats. Careful allowance must be made 

 * or t ^ le P ress ures which are to be withstood. 



Atmospheric pressure was first measured by a 

 famous experiment known as TorricellVs (tor-ri-chelly) ex- 

 periment. TorricelH (1608-47) was a pupil of Galileo, the 

 famous Italian scholar of whom you have already read. 

 Galileo himself noted that suction-pumps were unable to 

 lift water more than about thirty-two feet, and he probably 

 suspected that this rise was due to atmospheric pressure. 



