BRIDGMAN. — HIGH PRESSURE EXPERIMENTING. 629 



space. Reference to Figure 2 will show that such a packing must 

 leak as soon as the pressure in the liquid reaches the pressure applied 

 initially to the packing by the screw. 



We proceed to the detailed consideration of the various parts of 

 the apparatus. 



The Mechanism for Moving the Piston. 



This mechanism, of course, may be anything that will furnish a 

 force of the required intensity and exert it over a long enough distance. 

 It has been usual in previous high pressure work to use a screw to 

 drive the piston. When the pressure to be produced becomes high, 

 the screw becomes very inefficient, and it is highly desirable to replace 

 it with a hydraulic press. In my early work up to 6000 kgm., the 

 piston (I inch in diameter) was driven by a screw. This screw had a 

 pitch of 8 threads to the inch, and needed a six foot wrench to turn it. 

 Its efficiency was less than 5%. Furthermore, even when made of 

 nickel steel, the screws, which were one inch in diameter, finally 

 broke down and had to be replaced. The hydraulic press which I 

 have been using for the last six years has never given the slightest 

 trouble. To avoid bulky apparatus, it is desirable to actuate the 

 press with liquid at a fairly high pressure. For this purpose a pump 

 of the Societe Genevoise is convenient, giving 1000 kgm/cm- with a 

 lever. The diameter of the piston of the hydraulic press is 2j inches, 

 which permits, therefore, of a pressure of 25000 kgm/cm^ on a | inch 

 piston, the size usually used. The barrel of the press is made of mild 

 steel, 4 inches in outside diameter. The piston is threaded over the 

 entire length and provided with a heavy nut by which the piston 

 may be maintained in any desired position, even when the pressure 

 on the low end is relieved. This arrangement has proved an indis- 

 pensable convenience in operation. Opposed to the main hydraulic 

 press is a smaller press with a f inch piston, connected to the larger 

 piston by tie rods and a yoke pressing against the nut. By this 

 auxiliary press the piston of the large press can be rapidly returned 

 to its initial position after completion of a stroke. 



It is essential that the press be accurately constructed so that the 

 thrust on the high pressure piston shall be exactly centered; other- 

 wise the piston will buckle. 



The packing on the low pressure piston of the press embodies the 

 principle shown above. It might be possible to use a cup leather, 



