330 PROCEEDINGS OF THE AMERICAN ACADEMY. 



suspended directly from the upper end of this piston. This piston was 

 kept in constant rotation by a small motor so as to avoid friction. The 

 other freely moving piston, 5/16 inch in diameter, was in direct hy- 

 drostatic communication with the 1/4-inch piston, and at its upper 

 end pressed directly against the springs to be calibrated. (This 5/16- 

 inch piston could be rotated by hand as occasion required to destroy 

 friction.) From the weight on the 1/4-inch piston, and the areas of 

 the 1/4 -inch and the 5/16-inch pistons, the thrust on the springs 

 could be calculated. The area of the 5/16-inch piston was found in 

 the same way as that of the 1/16-inch piston. The method has all the 

 advantages of the discarded method of the direct application of weights, 

 namely, complete freedom from all elastic effects and hysteresis, and in 

 addition permits very much more convenient and flexible application 

 of pressure. 



The procedure of the calibration was to place a weight on the 1/4- 

 inch piston, completely depressing it. The piston was then floated 

 again by raising the pressure to the equilibrium value with the Cail- 

 letet pump. The 5/16-inch piston was then rotated to destroy fric- 

 tion, and the deflection of the spring read. Eleven such steps were 

 made with increasing and decreasing pressure, making twenty-two 

 steps in all. The same weights were used in all the calibrations, so 

 that the results were strictly comparable. The pressure exerted by 

 the fluid, as given by the gauge of the Geneva pump, was also recorded 

 as a check. The accuracy of the other readings was so great, however, 

 that these check readings could never be used. 



Calibration with this device was first made to find whether the 

 gauge had any error of position, since it was generally calibrated verti- 

 cally, but used horizontally. This could evidently be done very simply 

 by changing the position of the cylinder with the 5/16-inch piston, 

 a change in the calibrating procedure which could not be made so 

 simply when weights were directly applied. The result of the calibi-a- 

 tion in the horizontal position showed no detectable error due to this 

 change of position. Of course no such error was to be expected, since 

 all the parts were very stiff in comparison with their weights. 



A second result of the calibration was that the springs show no 

 elastic after-effects. By this is meant the gradual creep after applica- 

 tion of a load, and gradual recovery after removal. The effect is 

 generally most pronounced at the two ends of the pressure range. 

 These springs, however, showed no tendency to yield viscously under 

 the maximum stress, and never showed any wandering of the zero after 

 release of pressure. 



A third result of the calibration was that the springs do not follow 



