BRIDGMAN. — A DETERMINATION OF COMPRESSIBILITIES. 265 



the aluminum, although, because of the smaller size of the total effect, 

 one would expect greater percentage variation from the particles of 

 grit. Probably the improvement is due to the increased familiarity 

 with the method, which seems capable of giving accurate results. A 

 straight line through the observations, discarding the four worst, was 

 computed by least squares, giving as the linear compressibility 

 1.722 X 10 — 7 , and the cubic compressibility 5.166 X 10 — 7 kgm. per 

 sq. cm. Table II shows the differences between the observed and the 

 computed values. The four starred points are the ones discarded in 

 the computation. The probable error of a single observation, except- 

 ing the four discarded ones, is 2.3, less than \ per cent at the higher 

 pressures. The probable error of the compressibility is tV per cent, 

 which therefore does not vary more than this from constancy through- 

 out the pressure range. No set was observed in this piece of steel on 

 the first application of pressure, which is perhaps evidence of the free- 

 dom from internal strain, and to a less degree evidence for equal 

 compressibility in all directions. 



An attempt was made to get some light on the possible magnitude 

 of differences of compressibility in different directions by the follow- 

 ing method: Two strips were cut from a very homogeneous piece of 

 f in. (1.59 cm.) Bessemer boiler plate, respectively along and per- 

 pendicular to the direction of rolling; these were turned down to 

 f in. (0.95 cm.) like the other test pieces of steel or aluminum, and 

 the compressibility of each determined. The results are given in 

 Tables III and IV. The compressibility of each was calculated by 

 least squares, discarding only one observation from each set. The 

 probable error of a single observation is approximately the same in 

 either set, t 7 q per cent at the higher pressures. The probable error of 

 the compressibility in either case is about ^o per cent. The compressi- 

 bility of the lengthwise piece was 5.298 X 10 — 7 , and of the transverse 

 5.303 X 10 — 7 , agreeing within the limits of error. No claim is made 

 that this settles the question of the equal compressibility of metals in 

 all directions. Doubtless with metals of different character there are 

 internal strains left from working that would produce such a difference. 



There are only a few other direct determinations of the compressi- 

 bility of steel. Amagat 2 measured the change of length by an electric 

 contact device, but does not publish his data. He states that the re- 

 sults agree with a determination by an indirect method involving the 

 theory of elasticity and gives 6.8 X 10 — 7 as the best value. Richards,3 



2 Amagat, C. R., 108, 1199 (1888). 



3 Richards, loc. cit., p. 50. 



