Alii PUMPS AND APPARATUS. 223 



1 1, is usual to measure it by the compression of a certain 

 quantity of air contained in a tube of sufficient strength. It 

 consists of a stout glass tube, the bore about one tenth of 

 an inch in diameter, closed at one end, and bent up at the 

 other, having a ball about twice the capacity of the tube, 

 with a small hole on the side or top, and which is half filled 

 with mercury ; the tube is attached to a silvered plate, 

 graduated to the various pressures, which are represented 

 in pounds to the square inch ; the scale and tube are in- 

 closed in a stout glass shade, two inches in diameter, and 

 about ten inches long, cemented to a brass cap, and screw 

 ing on a large stop-cock, having a stout flange at its side, 

 with holes for screws for supporting the whole from the 

 wall, or posts, etc. ; at the lower end of the stop-cock is a 

 coupling box, for connecting a pipe communicating with the 

 apparatus or engine, the stop-cock closes the communication 

 when required ; under the ordinary pressure of the air the 

 mercury will stand at on the scale, near the bottom of 

 the tube ; under a greater pressure the mercury will be 

 forced into the tube, and when occupying one half the 

 original space, will indicate the additional pressure of our 

 atmosphere, or fifteen pounds to the square inch ; when 

 occupying one third, a pressure of thirty pounds ; one fifth, 

 ninety pounds, etc. Price, $12.50. 



Vacuum Gauge, or Indicator. The arrangement of this y 

 instrument is the same as represented in Fig. 236, in regard 

 to the exterior mounting, but having the tube entirely filled 

 with mercury under the pressure of atmosphere, and on the 

 removal of the pressure the mercury falls, and the approach 

 to a vacuum is .indicated by the scale, which is graduated to 

 correspond to the common barometer ; thirty inches being 

 assumed as a vacuum. Price, $12.00. 



