630 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



but this would probably last for only a short time, for my experience 



has been that even so low a pressure as 1000 kgm. is sufficient to 



rapidly mechanically disintegrate the 

 fibrous structure of the leather. Fig- 

 ure 3 shows the packing. The end 

 piece A with stem B plays in the hole 

 ( ' in the end of the piston. The pack- 

 ing is a disc of | inch soft rubber be- 

 tween two washers of | inch red fibre. 

 The rubber washers will wear away in 

 time, but the fibre washers will have 

 then become so adapted as to give the 

 necessary tightness. In six years of 

 constant use, this packing has been 

 renewed only once, and then only as a 

 matter of precaution, when the press 

 was dismantled for another purpose. 

 In order to make absolutely sure that 

 the end of the stem B is always free 

 from stress it is well to drill a small 

 lateral hole at the bottom of C to allow 

 any liquid to flow away that might 



percolate around the packing when it is relaxed, as it is when there is 



no pressure behind it. 



Figure 3. Shows a section of 

 the cjdinder of the hydrauKc 

 press with the packing on the 

 end of the piston P. Pressure 

 up to 1000 kgm. per sq. cm. is 

 exerted on the piston by means 

 of a liquid at L. The scale of 

 the diagram is } actual size. 



The High Pressure' Piston. 



It has been found best to make this in two distinct parts; a piston 

 proper by which pressure is transmitted from the press, and a packing 

 plug driven by the piston into the cylinder. The piston proper is a 

 cylindrical piece of steel with perfectly plane ends. It is made of tool 

 steel, first turned between centers to the approximate dimensions, 

 then hardened glass hard and left Avith the temper undrawn, ground 

 to the final size between centers, and finally the center marks ground 

 out, leaving the ends plane. If the center holes are not ground out, 

 the piston is much more likely to crack. For pressures up to 15000 

 kgm. any high grade of carbon tool steel is good enough. For higher 

 pressures it will be well to use one of the special tool steels that admit 

 of being made especially hard. I have found chrome or silicon steels 

 very suitable. One of these broke at 50,000 kgm/cm^. It has been 



