154 



Mr. J. T. Bottomley on a Practical 



line which is at the middle of the long branch of the U (see 

 fig. 1). The longer tube is numbered upward and downward, 

 and the shorter downwards, from the zero line, and the 

 mercury is filled in so as to stand at the zero in both branches 

 when there is no difference of pressure, and thus the sum of 

 the readings of the two tubes is equal to the difference of 

 pressures when any difference of pressure exists. 



The pressure apparatus consists of a simple pressure-syringe 

 which forces air into a small air-bag of india-rubber fortified 

 with canvas. The air-bag is placed between two boards, which 

 are connected by a leather hinge and pressed together by 

 means of a nut which works on a wooden screw. The air- 

 bag is also connected by means of a T-tube with the three- 

 way stop-cock t f ; and, by means of this stop-cock, the air 

 presses both on the liquid in the volume-gauge, and on the 

 shorter column of the manometer. The india-rubber tubes 

 used for these connexions require to be strengthened with 

 canvas to resist the pressure. 



The form of volume-gauge shown in fig. 3 is designed for 

 use at very high temperatures. It is made in two parts, 

 which are connected together at the cup e (shown enlarged, 

 fig. 4) . The bulb and tubes c, d, and b Fig. 4. 



are made of hard Bohemian glass; the 

 remaining part of the gauge is of German 

 glass or English flint glass. The stopper 

 of the cup e is made to fit the throat of 

 the cup closely, and just below the throat 

 an enlargement / is blown out, through 

 which the elongated part of the stopper 

 passes. The stopper is fastened air-tight 

 into the cup with German " Siegelwachs ;" 

 and the object of the enlargement is to 

 furnish a cushion of air which prevents 

 the liquid of the volume-gauge from 

 coming in contact with the cement. The 

 making of this joint is a little trouble- 

 some, and it requires to be protected 

 against radiation from the hot source. 

 There are various stoppers and joints 

 well known, which prevent leakage in- 

 wards from without ; but it is much more 

 difficult to find an efficacious stopper 

 which will act against pressure from 

 within outwards. 



The thermometer-bulb is filled with 

 perfectly pure dry air, and it is desirable to have the bulb 



