BRIDGMAN. 



A DETERMINATION OF COMPRESSIBILITIES. 



273 



D 



C 



discrepancies of 3 per cent. Finally, the correction for the glass must 

 be determined from the theory of elasticity, assuming uniform com- 

 pressibility in all directions. The 

 difficulty of obtaining glass free 

 from internal strain makes the 

 validity of this assumption at least 

 doubtful. Many anomalous results 

 may be explained by this effect. 

 Thus in one case 8 an actual in- 

 crease of the internal volume of the 

 piezometer under hydrostatic pres- 

 sure has been recorded. On the 

 other hand, the great homogeneity 

 of steel makes its uniform com- 

 pression apriori more probable, 

 and here the probability has been 

 greatly increased by an experi- 

 mental proof of the uniformity of 

 strain in a piece of rolled steel plate, 

 of the same grade of steel as that 

 used in the mercury piezometer. 



The method is essentially a re- 

 vival of one used by Perkins 9 in 

 1825. Possibly the bad results 

 obtained by Perkins, which were 

 250 per cent too large, accounts 

 for the subsequent neglect of the 

 method. Several slight modifica- 

 tions were suggested by Professor 

 Sabine, however, so that it has 

 been possible to obtain very satis- 

 factory results. The method con- 

 sists essentially in observing the 

 extent to which a freely moving 

 piston is pushed into a cylinder con 



G 



Figure 8. Piezometer for deter- 

 mining the compressibility of mer- 

 cury. C, containing cylinder of 

 steel; G, mercury; P, easily mov- 

 ing piston; D, movable brass ring by 

 winch the displacement of the piston 

 is measured. The packing of molasses 



and glycerine is placed at E. The 



taining the liquid to be examined, piezometer is closed at the lower end 



bvthe application of hydrostatic by the steel plug A, held in place by 



J ,, , • n i the screw B. I he crack at h is filled 



pressure all over the exterior ot the ^h solder, 



piston and cylinder. The arrange- 

 ment used is shown in Figure 8. The containing cylinder C is of 



8 M. Schumann, Wied. Ann., 31, 22 (1887). 



9 Perkins, Phil. Trans. Royal Society, London, p. 324 (1819-1820). 



VOL. XLIV. — 18 



