THE USE OF WEATHER INSTRUMENTS 



183 



taught that "Nature abhors a vacuum." Galileo remarked, 

 when observing the pump, that nature's abhorrence seemed 

 to stop at 32 ft. His experiments in search of the explanation 

 were ended by his death in 1642. Torricelli, his pupil, at 

 Galileo's request, continued the investigation. Torricelli 

 concluded that air pressure had something to do with the 

 action of the pump. He remembered that mercury is 13.5 

 times as heavy as water; he reasoned that if the air pressure 

 raised water 32 ft. in the pump, it would raise or support 

 mercury Ks-5 as m gh> O r about 29 in. He experimented 

 and proved his theory. One of his experiments, first per- 

 formed in 1643, is often repeated today. 



Exercise 47. Torricelli's Experiment 



Secure a glass tube about 36 in. long, closed at one end. Fill it with 

 mercury, working over a pan to catch spilled mercury. Pour the 

 remaining mercury into a glass or iron cup 

 (Fig. 129). Place your finger firmly over the 

 top of the glass tube and invert the tube care- 

 fully, putting the open end into the mercury 

 in the cup. Remove the finger. What hap- 



pens? There is likely to be considerable air 

 in the mercury column, which quickly rises as 

 bubbles to the surface in the tube. To remove 

 this air; slip your finger over the open end of 

 the tube. Lift the tube from the cup and 

 stand it upright. Add mercury till the tube 

 is full. Invert it again in the cup of mercury. 

 Measure the height of the mercury in the tube 

 above the surface of the mercury in the cup. 

 This should be about 74 or 75 cm., or about 29.9 

 in. at sea level, becoming less at higher altitude. 



FIG. 129. Torri- 

 celli's experiment. The 

 tube is shortened very 

 much in the illustra- 

 tion. 



211. Measuring the Atmospheric Pressure. The pressure 

 of the air on the surface of the mercury in the cup supports 

 the mercury in the tube. The height of the mercury column 

 thus shows the weight, or pressure, of the air. The Torricelli 

 tube, and the more elaborate barometer, are so often used 

 to measure the atmospheric pressure that we have come to 



