PRESSURE OF THE ATMOSPHERE. 231 



instead of small bottles, very tall vessels might be used; 

 but if the height of the vessel were to exceed 10 m , 

 which is about the height of a house of moderate size, 

 the pressure of the air would be incapable of supporting 

 the whole of the liquid. This may be proved by the ap- 

 paratus represented in fig. 160. A number of glass tubes 

 are joined by connecting pieces of brass, and form a 

 tube of about 12 m length, which is fixed to a wall 

 sufficiently high, or kept in a vertical position by 

 means of a scaffolding specially erected for the pur- 

 pose. Both ends are provided with stop-cocks, the 

 upper end also with a funnel ; the lower end dips in a 

 vessel with water. At first both stop-cocks are open, 

 and the water in the tube is hence at the same level as 

 that in' the vessel, which is marked in the figure. The 

 lower stop-cock is then closed, the tube is filled with 

 water through the funnel, and when it is full the 

 upper stop-cock is closed. When the lower stop-cock 

 is opened, some of the water will run out of the tube 

 into the vessel, but a column of water will remain in 

 the tube, having a length of about 10 m , measured from 

 the level of the water in the lower vessel to the top of 

 the column, as shown in the figure. The pressure of 

 \the atmosphere is capable of supporting a column of 

 water 1 O m high, or, the pressure of a column of water of 

 \this height is equal to the pressure of the atmosphere. 



The pressure of the atmosphere may be more con- 

 veniently measured if, instead of water, mercury is 

 employed; mercury is much heavier than water, and a 

 smaller column will therefore balance the atmospheric 

 pressure. A glass tube closed at one end, 80 cm long 

 and about 5 mm wide, is filled with mercury ; the open 



