632 PROCEEDINGS OF THE AMERICAN ACADEMY. 



up ready for experiment, the other for use in giving the prehminary 

 seasoning. I describe first the plug for regular use. Figure 5 shows 

 the dimensions. It is particularly important that the corners at A 

 should be left slightly rounded. The plug may be made an easy fit, 

 say 0.001 of an inch too small, for the hole in the cylinder. The plug 

 is further provided with a washer of copper, B, yg of an inch thick, a 

 force fit for the hole. For convenience in handling, it is well to solder 

 the washer B to the plug. Above B is the packing washer, C. | of 

 an inch is thick enough for this rubber washer if only a few strokes of 

 the piston are to be made, but since the enormous friction rapidly wears 

 away the rul)ber, the packing should be made thicker if longer use is 

 contemplated. It is an advantage to' keep the packing as thin as 

 possible, for in this way the total friction is reduced. Above the 

 packing is another copper washer, D, identical with the lower one, 

 and above this a washer, E, of the same dimensions, of soft Chrome 

 Nickel steel. Finally, above the soft steel washer, is a hardened steel 

 ring, F, against which the piston bears. This ring is likely to crack. 

 If pressures not over 12000 kgm. are to be used, the ring may be made 

 of tool steel, hardened, and drawn to a black. But if pressures to 

 25000 are to be used, the ring must be left glass hard, and one may 

 expect to renew this ring with each new setting up of the apparatus. 



The packing plug just described is one of the few parts of the appa- 

 ratus almost certain to give way after long use. Fortunately the re- 

 sults of failure are nothing more serious than the projection of the 

 severed stem against the piston, and sometimes, though not usually, 

 leak. Failure takes the form of separation of the stem from the head 

 at the corner A by the " pinching-off " ■'^ effect. The grade of steel 

 used for the plug is therefore, important; a peculiar sort of toughness 

 is necessary. I have found best for this a Krupp Chrome Nickel steel, 

 grade E. F. 60.0. The New York agents are Thos. Prosser and Son, 

 15 Gold St. It is strange that another grade of steel with a higher 

 tensile strength, and better adapted for the construction of the cylin- 

 ders, is not so good for the plug. The plug is to be hardened in oil, 

 and the temper left undrawn. The degree of heating during the 

 hardening is important, slight differences having a great effect on the 

 resistance to the " pinching-off " effect. The precise temperature of 

 heating can best be told by experiment. 



The size of the rubber washers used for packing is also important; 

 these should be as much larger than the hole as can conveniently be 



1 P. W. Bridgman, Phil. Mag., 24, 68 (1912). 



