170 



WELLS'S NATURAL PHILOSOPHY. 



How was the 

 conclusion of 

 Torricelli fur- 

 ther verified ? 



ricelli expected, and he soon FiG, 153, 



perceived the true cause of the 



phenomenon. The weight of 



the atmosphere acting upon 



the surface of the mercury in 



the vessel, supports the hquid 



in the tube, this last being 



protected from the pressure of 



the atmosphere by the closed 



end of the tube. 



379. The fact 

 that the col- 

 umn of mer- 

 cury in the 

 tube was sustained by the 

 pressure of the atmosphere, 

 was further verified by an ex- 

 periment made by Pascal in 

 France. He argued, that if 

 the cause which sustained the 

 column in the tube was the 

 weight of the atmosphere act- 

 ing on the external surface of 

 the mercury in the cistern, 

 then, if the tube was trans- 

 ported to the top of a high 

 mountain, where a less quan- 

 tity of atmosphere was above 

 it, the pressure would be less, 

 and the length of the column less. Tlie experiment was tried by carrying 

 the tube to the top of a mountain in the interior of France, and correctly 

 noting the height of the column during the ascent. It was noticed that the 

 height of the column graduaUy diminished as the elevation to which the 

 instrument was carried increased. 



The most simple way of proving that the column of mercin-y contained in 

 the tube, as in Fig. 158, is only balanced against the equal weight of a column 

 of air, is to take a tube of sufficient length, and having tied over one end a 

 bladder, to fill it up with mercury, and invert it in a cup of the same hquid; 

 the mercury will now stand at the height of about 30 inches ; but if with a 

 needle we make a hole in the bladder closing the top of the tube, the mer- 

 cury in the tube immediately falls to the level of that in the cup. 



These experiments by Torricelli led to the invention of the 

 Barometer. It was noticed that a column of mercury sus- 

 tained in a tube by the pressure of the atmosphere, the tube 

 being kept in a fixed position, as in Fig. 159, fluctuated from 

 day to day, within certain small hmits. This effect waa 



IIow did the 

 experiment of 

 Torricelli lead 

 to the inven- 

 tion of the Ba- 

 rometer ? 



