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PROCEEDINGS OF THE AMERICAN ACADEMY. 



weight to the piston, and then vary the pressure until equilibrium was 

 produced. Setting on this equilibrium pressure was made more diffi- 

 cult by the fact that pressure always showed a tendency to fall after 

 an increase, and to rise after a decrease, a fact that may be explained 



TABLE I. 



Comparison of Two Absolute Gauges against the Same 

 Mercury Gauge. 



by thermal effects of compression, but is more probably due to elastic 

 after effects in the containing steel vessels. It may be concluded, 

 therefore, from the agreement of these comparisons, that even if all 

 the error is in the absolute gauge and none in the mercury resistance, 

 that this type of gauge is good to about -^ per cent. 



The comparison with mercury gauges also furnished an estimate of 



