328 PROCEEDINGS OF THE AMERICAN ACADEMY. 



able with the new device was, however, sufficient for all requirements. 

 There is an added advantage in that the stroke of the piston over the 

 entire pressure range may be very much decreased. The stroke of 

 12 mm. in the previous work was reduced to J. 5 mm. in this form. 

 This ensures greater strength at the upper end where the piston pro- 



FiGURE 3. The springs with which the thrust on the piston is measured. 



jects unsupported from the cylinder, and also ensures greater accuracy, 

 the piston always playing in approximately the same part of the 

 cylinder. 



Several forms of spring were tried. The form finally adopted as 

 giving the great stiffness desired without inconvenient bulk was that 

 of a saucer. Two of these saucer-shaped springs were used, placed rim 

 to rim, and the thrust applied at the center (see Figure 3). This ar- 

 rangement has the advantage of avoiding all friction when the circum- 

 ference of the saucer increases under pressure, since here the two 

 saucers support each other and move together without slipping. 



The choice of steel is of great importance. The most suitable of the 

 many kinds tried was one manufactured by the Halcomb Steel Co. in 

 the form of sheets. 



Much experimentation was necessary before the best dimensions and 

 the best heat treatment of the springs was found. The sheet from 

 which the springs were cut was 0.105 inches thick. This was cut into 

 discs 2 1/2 inches in diameter, with a 1/4-inch hole through the cen- 

 ter. The disc was then turned on one flat face so as to decrease regu- 

 larly from 0.105 inches at the center to 0.035 inches at the edge. It 

 was given the saucer shape in a die while hot, the depression at the 

 center being about 1/4 inch. The greatest care was necessary in hard- 

 ening to prevent warping. This was conveniently done by heating to a 

 bright red heat in a bath of molten lead so as to secure a uniform heat, 

 and then quenching by plunging edgewise into water. The temper 

 was drawn by heating to 370° C. in oil. The temperature was impor- 

 tant : 360° C. was too low. Finally, after tempering, the springs were 

 ground fiat on the edge to provide a bearing surface against each 

 other. Warping due to hardening was most distinctly shown by varia- 



