THE BAROMETER. 



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281 j 



about 28 inches. He procured a glass tube, A B (fig. 1). more than 30 inches 

 in length, open at one end, A, and closed at the other end, B. Placing this 

 tube in an upright position, with the open end upward, he filled it with mer- 

 cury, and applying his finger to the end A, so as to prevent the escape of the 

 mercury, he inverted the tube, plunging the end A into a cistern, C D (fig. 2), 

 containing mercury, the open end A being below the surface F of the mer- 

 cury in the cistern, and no air having been allowed to communicate with it. 



Fig. 1. 



A 



Upon removing the finger, therefore, the mercury in the cistern came in imme- 

 diate contact with the mercury in the tube. Immediately the mercury was 

 observed to subside from the top of the tube, and its surface gradually to de- 

 scend to the level E, about 28 inches above the mercury in the cistern. This 

 result was what Torricelli anticipated, and clearly showed the absurdity of the 

 supposition that nature's abhorrence of a vacuum extended to the height of 32 

 feet. Torricelli soon perceived the true cause of this phenomenon. The at- 

 mospheric pressure acting upon the surface F, while the surface E was pro- 

 tected from this pressure by the closed end B, of the tube, supported the weight 

 of the column E F. This pressure was transmitted by the liquid mercury in 

 the cistern from the external surface F, to the base of the column contained in 

 the tube. 



This experiment and its explanation soon became known to philosophers in 

 every part of Europe, and, among others, it attracted the notice of the cele- 

 brated Pascal. In order to subject the explanation of Galileo to the most se- 

 vere test, Pascal proposed to transport a tube of this kind to a great elevation 

 upon a mountain, and argued that, if the cause which sustained the column in 

 the tube were the weight of the atmosphere acting upon the external surface 

 of the mercury in the cistern, then it must be expected that if the tube was 

 elevated, having a less and a less quantity of atmosphere above it, the column 

 sustained by the weight of this incumbent atmosphere must suffer a correspond- 

 ing diminution in height. He accordingly directed a friend residing in the 

 neighborhood of a mountain called Pays de Dome, near Auvergne, to ascend 

 that mountain, carrying with him the apparatus already described. This was 

 accordingly done, and the height of the column noted during the ascent. Con- 



