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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



by designing the packing in such a way that tlie pressure in it is main- 

 tained automatically, by the liquid pressure itself, at a level higher 

 by a fixed percentage than that in the liquid. The principle will be 

 clear from figure 1, which illustrates the packing on the end of a 

 piston by which pressure is generated. A piston packed in this way 



Figure 1. — Application to a piston of the principle by which the pressure in the packing 

 is automatically maintained at a pressure greater by a fixed percentage than the pressure 

 in the liquid. Leaks therefore cannot occur. 



cannot leak, so that all one needs to do to produce any desired pres- 

 sure is to push the piston into the pressure vessel with the necessary 

 force. The force driving the piston is most advantageously obtained 

 from a hydraulic press or some other form of hydraulic intensifier. 

 "Any desired pressure" is, however, obviously subject to several 

 limitations, in particular the strength of the containing vessel and of 

 the piston. It might perhaps be thought at first that the strength 

 of the containing vessel could be increased indefinitely, merely by 

 making the walls of the vessel of unlimited thickness. This unfortu- 

 nately is not so, for even an infinitely thick vessel has only a finite 

 strength. The reason for this is that stress and strain are concen- 

 trated at the inner parts, so that the outer parts do not do their 



