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PROCEEDINGS OF THE AMERICAN ACADEMY. 



more than one part in 400,000 could be detected. On a previous 

 occasion a piece of drawn copper had been found to suffer no increase 

 of weight of one part in 1,800,000. It may be confidently expected, 

 therefore, that ordinary commercial bar metal shows no considerable 

 porosity. Amagat's result was probably due to flaws in the casting. 



In Figure 9 the observed proportional changes of volume of mercury 

 measured from an arbitrary zero, as in the case of the determination 



1000 



2000 



60O0 



70OO 



Figure 9. The proportional change of volume of mercury, as determined 

 with the piezometer of Figure 8, plotted against pressure. 



of the compressibility of rods, are plotted against pressure, measured 

 in the usual way with a mercury resistance. The maximum ordinate 

 corresponds to a displacement of the piston of 1.5 cm. Results ob- 

 tained with a preliminary piezometer, not so well made as the final 

 one, agree with the curve given within the somewhat larger limits of ex- 

 perimental error. The principal source of error seems to be the in- 

 clusion of minute air bubbles. Measurement from an arbitrary zero, 

 determined by backward extrapolation as above, removes this as a 

 consistent source of error, but the measurement of the actual dis- 

 placement becomes irregular from the lack of certainty with which 

 the piston is returned after release of pressure to the initial position by 

 the comparatively feeble expansive action of the bubble of air. All the 

 precautions described above to remove this bubble appear necessary. 



