322 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Amagat were the smallness of the piston exposed to pressure (1/16 inch), 

 and the fact that the pressure cylinder in which this piston moved was 

 exposed to pressure on the external as well as on the internal surface, 

 so that the effect of the increasing pressure was to decrease the inter- 

 nal bore of the cylinder and 

 so decrease the leak. The 

 application of pressure to the 

 outside of the cylinder was 

 made through the medium of 

 the packing.^ The figure 

 referred to shows that the 

 disposition of the packing 

 was such that the cylinder 

 was exposed externally to 

 pressure over the lower end, 

 and to none at all over the 

 upper. At high pressures the 

 effect of such an arrangement 

 is invariably to pinch off the 

 cylinder, the packing forcing 

 its way into the cylinder and 

 separating the upper from the 

 lower end. Two things are 

 necessary to produce this 

 effect : external pressure and 

 non-support of one of the 

 ends. The action is similar 

 to that of a roll of putty 

 which, when squeezed in the 

 hand, will ooze out sidewise if the ends of the roll are left free, but may 

 be prevented by compressing the ends of the roll between the fingers 

 of the other hand. 



In the new form of gauge the two novel features, smallness of piston 

 and a cylinder exposed to pressure on the outside, have been retained, 

 but the manner of applying the packing has been so changed as to 

 avoid the pinching-off effect. The new gauge is shown in Figure 1. 

 The cylinder AB is placed lower down than in the former gauge, so 

 that now it is in the part of the metal subjected to hydrostatic pressure 

 only. The packing, which now takes the form of a cone of soft steel, 

 D, is placed above the upper end of the cylinder. This new form 



Figure 1. The absolute gauge; shows 

 the cylinder of the gauge and the method of 

 mounting it in the containing vessel. 



1 See Figure 2, p. 205, loc. cit. 



