90 



PHYSIOGRAPHY. 



[chap. 



dosing the open end with the thumb, as shown in the 

 right-hand figure of Fig. 21, invert it in a basin of mercury 

 so that the open end may dip beneath the Hquid ; it will 



then be found that the mer- 

 cury falls for a short distance 

 in the tube, but that a column 

 about 30 inches in length re 

 mains suspended, as shown 

 in the left-hand figure. Tor- 

 ricelli argued that this column 

 must be supported by the 

 pressure of the external at- 

 mosphere on the surface of 

 the mercury ; the downward 

 pressureof the columnof mer- 

 cury being exactly balanced 

 by the upward pressure of 

 the atmosphere transmitted 

 through the quicksilver. In- 

 deed, if we admit air by 

 making a hole in the top 

 of the tube, the column im.- 

 mediately falls, since it is 

 then pressed down by the 

 atmosphere above; but when 

 the tube is closed there is no 

 atmospheric pressure on the 

 top within the tube, for the 

 space above the column of mercury is completely empty, or 

 rather contains only mercurial vapour, whence it is called 

 the Torricellian vacuum. Since the column of mercury in- 

 side is balanced by the atmosphere without, it follows that if 

 we know the weight of the mercury we know also the weight 



Fig. 21. — TorricelU's experiment. 



