THE WATER-HAMMER. 



801 



~A A 



air in the retort will be expelled. The india-rubber 

 tube may now be closed by a pinch-cock and the lamp 

 immediately removed ; the space within the retort no 

 longer contains any air, but is filled with steam, of which 

 the pressure is at first the same as that of the atmo- 

 sphere, but as the temperature decreases the pressure also 

 becomes less, and finally there 

 will be in the retort only very 

 rarefied steam exerting an in- 

 , considerable pressure. 



In the same manner the pre- 

 viously-mentioned water-ham- 

 mer is exhausted of air. Where 

 the small point appears near 

 the end there was a short narrow 

 tube through the opening of 

 which the air has been driven 

 out and which was then closed 

 with the blow-pipe. When the 

 apparatus is inverted so that 

 some of the water strikes against Fm ' 393 <* real 

 the extremity of the tube, or against another portion of 

 the liquid contained in it, a sharp sound is produced 

 similar to that which accompanies the condensation of 

 steam in water. It resembles so much the shock of two 

 solid bodies that, on hearing the sound for the first time, 

 it appears as if the glass had been cracked. The sound 

 is particularly sharp if the apparatus is either held in 

 the position ^4, fig. 393, and moved quickly in the 

 direction of the arrow, so that the water in the bulb 

 strikes against that in the tube, or in position B, and 



3F 



