102 



SCIENCE OF COMMON THINGS. 



Pascal is experiment. 



Invention of the barometer. 



end, so as to prevent its escape, he inverted it, plunging the end into mer- 

 cury contained in a vessel. On removing the linger, he observed that the 

 mercury in the tube fell, but did not fall altogether into the cistern; it 

 only subsided until its surface was at a height of about 30 inches above 

 the surface of the mercury in the cistern. The result was what Torricelli 

 expected, and he soon perceived the true cause of the phenomenon. The 

 weight of the atmosphere acting upon the surface of the mercury in the 

 vessel, supports the liquid in the tube, this last being protected from the 

 pressure of the atmosphere by the closed end of the tube. 



647 How was the fact that the column of mercury was sustained by the 

 pressure of the atmosphere further verified ? 



By an experiment made by Pascal, in France. He 

 argued, that if the cause which sustained the column in 

 the tube was the weight of the atmosphere acting on 

 the external surface of the mercury in the cistern, then, 

 if the tube was transported to the top of a high moun- 

 tain, where a less quantity of atmosphere was above it, 

 the pressure would be less, and the length of the column 

 less. This was tried and found to be the case. 



648 How did these experiments lead to the invention of the barometer f 



It was noticed that when the ap- 

 paratus above described was kept 

 in a fixed position, the height of the 

 column fluctuated from day to day 

 within certain small limits. The 

 effect was of course to be attributed 

 to the variation in the weight of the 

 incumbent atmosphere, arising from 

 various meteorological causes. 



This led to the use of the tube and cistern 

 of mercury, arranged in the manner before 

 described (Fig. 26), for determining the changes 

 in the atmosphere, and consequently the cha- 

 racter of the weather. 



649 Explain more fully in what manner 

 the barometer can be used as a weather-glass ? 



When air is moist, or filled with 

 vapor, it is lighter than usual, and 

 the column of mercury stands low ; 

 when air is dry and free from vapor, 

 it is heavier than usual, and the mer- 

 cury stands high. Thus the baro- 



