222 BEXJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



Fig. 236. 



Fig. 233. 



Fig. 231. 



The Water Hammer. (Fig. 235.) This consists of a 

 glass tube usually about twelve inches long and one inch 

 in diameter, having three or four inches of water included 

 in it, above which a vacuum has. been formed, and then her- 

 metically sealed. On shaking such a tube vertically, the 

 water, rising a few inches and sinking suddenly to the bottom 

 of the tube, produces a sound like that arising from the 

 stroke of a small hammer on a hard body, whence the name 

 of this instrument, the action of which depends entirely on 

 the exclusion of the air, so that the water moves in a dense 

 mass. Price, 12 inch, $0.75 and $1.00. 



24 " $1.50. 



Pressure Gauge. (Fig. 236.) The pressure gauge is an 

 instrument to determine the pressure exerted in hydrostatic 

 or pneumatic machines, as the hydrostatic press, the air 

 pump, steam engine, etc. When the pressure is very great, 



