642 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The pressure of 6000 kgm. is still sufficient to project the fine stem 

 with considerable violence after rupture takes place, so that one must 

 be particularly careful not to yield to a fjilse feeling of security in 

 handling the plug during decreasing pressure. The insulating plug 

 as thus constructed is at present the part of the apparatus most likely 

 to fail. The mica washers eventually fail along a cone of shear 

 reaching from the hardened disc to the hole in the stem. Slipping 

 of the mica on the shear planes is likely to be accompanied by unsym- 

 metrical yielding of the rubber and the hardened disc, so that the fine 

 stem may be bent or even sheared oft' by the tilted disc. 



It would probably pay in designing new apparatus to experiment 

 with a form of insulating plug developed at the Geophysical Labora- 

 tory.* Soapstone instead of mica is the insulating material, and the 

 rubber washer is dispensed with, so that it is simpler in construction. 

 It has not been tested over so wide a range as the form described 

 above, but has given perfect satisfaction up to 8000 kgm. The form 

 described above has been used to 21000 kgm., although it would not 

 stand this many times. 



Valves. 



I have as yet not been able to construct satisfactory valves. How- 

 ever, it may be worth while to describe a substitute which has been 

 found useful for one particular purpose. In the absence of valves 

 the desired final pressure must be reached with one stroke of the piston. 

 But if the apparatus connected to the cylinder is of more than two or 

 three times the capacity of the cylinder itself, it will not be possible 

 to do this because of the compressibility of the transmitting liquid. 

 But since the greater part of the compression of a liquid occurs in the 

 first few thousand kilograms, it would in many cases be sufficient if an 

 initial pressure of a few thousand kilograms could be produced in the 

 cylinder before the stroke begins. This is accomplished by the use 

 of a small by-pass at the upper end of the cylinder, so situated that 

 it is just uncovered when tlie piston is withdrawn to its extreme posi- 

 tion. Initial pressure to the desired amount is produced in the appara- 

 tus by an auxiliary pump through the by-pass, which is then cut off 

 by the descent of the piston. It is necessary to make this by-pass very 

 minute, so that the rubber packing of the piston may not be blown out 

 through it as the piston passes by. One convenient way of doing this 



4 John Johnston and L. H. Adams, Amer. Jour. Sci., 31, 507 (1911). 



