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PROCEEDINGS OF THE AMERICAN ACADEMY. 



one, lie on a straight line within errors of reading. The one discordant 

 point represents a discrepancy of only 0.0003 mm., and no importance 

 is attached to it. The regularity of these measurements of the alumi- 

 num, made with the same apparatus as the measurements of the glass, 

 furnishes additional presumptive evidence, therefore, that the irregu- 

 larity of the latter is not due to errors of measurement, but is an 

 actual property of the glass. The lower line in Figure 7 was com- 

 puted by least squares, giving the relative compressibility of the 

 aluminum and the steel. From this and the known absolute com- 

 pressibility of the aluminum, the cubic compressibility of the steel was 



1000 



2000 



3000 4000 



5000 



<;< m >o 



7000 



Figure 7. Observed relative change of length of steel, and glass or alumi- 

 num. The ordinates give the change of length in millimeters, the total length 

 being about 8 cm. CD shows hard Jena glass; Q shows soft Jena glass, and O 

 the aluminum. 



found to be 4.74 X 10 — 7 , a value somewhat lower than the values 

 found directly for the other specimens of steel. Similarly, the other 

 lines of Figure 7, connecting relative change of length of glass and steel 

 with the pressure, were computed by least squares. The irregularity 

 of the results is too great to warrant the assumption of any other 

 than a linear relation, although the hard Jena glass in particular shows 

 a tendency toward the paradoxical behavior of higher compressibility 

 at higher pressures already remarked in the aluminum. From these 

 constants calculated by least squares, and. the compressibility of the 



