274 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Bessemer steel \ in. (1.27 cm.) diameter and 3 \ in. (8.89 cm.) long. 

 The piston P is -fe in. (0.16 cm.) in diameter, made in exactly the 

 same way as the piston of the absolute gauge described in a previous 

 paper. The piston accurately fits the .hole within 0.0002 or 0.0003 in. 

 (0.00051-0.00076 cm.). The cavity G, which is filled with mercury, 

 is \ in. (0.635 cm.) in diameter and 2 in. (5.08 cm.) long. The lower 

 end is closed with a plug of steel driven into place and soldered on 

 the outside at F and held additionally by the screw B. The piston P 

 is a slightly looser fit than that used in the absolute gauges, a few ounces 

 without rotation sufficing to displace it. The displacement produced 

 by pressure is indicated by the use of a sliding brass ring at D, ex- 

 actly as in measuring the change of length of rods. The piezometer 

 was filled by pouring recently distilled mercury through the small 

 hole at the top by a fine glass capillary. The inside of the piezometer 

 was first wet with a few drops of water to insure filling of all the 

 crevices. After filling in this way it was placed under an air pump 

 as an additional precaution against the inclusion of air. The whole 

 was now heated until the mercury rose from the top of the piston hole. 

 The piston, smeared to insure tightness with the same mixture of 

 molasses and glycerine used in the absolute gauge, was inserted and 

 follows the mercury down as it cools. The inside of the enlargement 

 at E was now smeared with molasses, and mercury was poured over 

 the whole to prevent contact of the molasses and the mixture of glyc- 

 erine and water transmitting the pressure. This packing of viscous 

 molasses very much improved the behavior of the piezometer, reduc- 

 ing the leak past the piston to a minimum. If, however, this packing 

 is used, its protection by the mercury is absolutely necessary, for other- 

 wise the glycerine diffuses through the molasses on each application of 

 pressure, rapidly changing the amount of liquid inside the cylinder. 

 The method of making the readings was to place the cylinder in 

 the pressure chamber and subject it to hydrostatic pressure all over. 

 By means of the freely moving piston this pressure is transmitted im- 

 mediately to the interior of the cylinder, the amount of motion of the 

 piston, and so the apparent loss of volume, being indicated by the dis- 

 placement of the ring D, which is measured after pressure is released 

 and the cylinder removed again from the pressure chamber. This 

 displacement, together with the cross section of the piston and volume 

 of the mercury, gives, therefore, the difference of compressibility be- 

 tween the mercury and the steel of the envelope. The volume of the 

 mercury was obtained by weighing, and the diameter of the piston 

 was measured with a Brown and Sharpe micrometer, the error here 

 not being more than 0.00005 in. on a total of 0.062 in., introducing 



