214 PROCEEDINGS OF THE AMERICAN ACADEMY. 



against the piston while the latter was in process of being ground to 

 size. The piston was too large to enter the hole except by forcing, 

 when 0.0001 in (0.00025 cm.) larger than the final size. . This allow- 

 ance is probably too much, but still probably not so high as to make 

 the error introduced here in the effective area as much as t*o per cent. 

 This method of measuring the diameter of a hole by testing against 

 plugs of known size is the method used by Brown and Sharpe them- 

 selves, and is probably the most accurate that we have, when it is pos- 

 sible to obtain the comparison plugs. The comparison of piston and 

 cylinder was easy in this case because all the work was done in the 

 machine shop of this laboratory. 



As preliminary work with this larger gauge, a Bourdon gauge by the 

 Societe Genevoise was calibrated to 1000 kgm., and showed a maxi- 

 mum error of 5 kgm. per sq. cm. Various liquids were used to trans- 

 mit pressure to the j in. piston, from vaseline which gave a barely 

 perceptible leak, to a thin mixture of water and glycerine, with which 

 the leak was so rapid that pressure could be maintained only with 

 difficulty. The indications of the gauge, as compared with the Bourdon 

 gauge, proved independent of the rapidity of leak, as they should. In 

 the use of the gauge, sensitiveness was secured as usual, by keeping 

 the piston in continual rotation. Made sensitive in this way, the 

 gauge was very much more sensitive than the Bourdon gauge, re- 

 sponding to about one part in 20,000 at 1000 kgm. 



Two high pressure gauges of the type described above were com- 

 pared with this J in. gauge at 1000 kgm. Pressure was kept constant 

 during the comparison by the rise or fall of the \ in. piston, which had 

 a long enough stroke to accomplish this. As was to be expected, the 

 larger piston proved more sensitive than the smaller ones. The cer- 

 tainty of rise or fall of the small pistons was made greater by observ- 

 ing them with the telescope of a cathetometer. The method of pro- 

 ceeding was to apply a constant weight to the small piston, and then 

 find the two weights on the large piston for which the small piston just 

 began to rise or fall. To accomplish this, the weight on the large 

 piston had to be changed by 0.4 kgm. with a total load of 300 kgm. 

 The mean of these two extreme values gives, therefore, the true equili- 

 brating weight to certainly T \ per cent, and probably much better than 

 this. 



From the effective area of either piston found in this way, and the 

 measured diameter, the size of crack between piston and cylinder can 

 be computed. It turned out to be 0.0001 in. (0.00025 cm.) for one 

 gauge, and 0.0003 in. (0.00075 cm.) for the other. This was roughly 

 verified by the more rapid leak shown at higher pressures by the latter 



