Fig. 1 represents the gauge secured to small ordnance, the gun shown in cross section.  Fig. 2 represents face view of the gauge and indicator, exposing a vertical section through the hydraulic portion of the gauge, on line 3 and 4 of Fig. 1.  The same principles of reduction of high pressure are used in this gauge as in Shaw’s hydraulic gauge.  It will be observed that a solid steel piston, E, in the cylinder, A, is provided with a plunger on its under side, which comes in contact with an elastic packing, D; the plunger may stand as 1 to A 1,000, or as 1 to A 100, in point of area of exposed surface, as compared with the large piston head, as desired.  Assuming the proportions to be 1 to A 1,000, the 1,000 lb. pressure on the plunger means only 1 lb. pressure in the fluid chamber, above piston head, E, and this greatly reduced pressure is now susceptible of measurement by any of the ordinary light pressure instruments for measuring pressures.  All the passage ways connecting to dial gauge, R, with the fluid chamber above piston, E, are filled solid with fluid, permitting no air spaces that can be avoided.  The steel plug, L, that forms a passage way between the fluid chamber and the dial gauge, is provided on one side with a small screw hydraulic pump, with a reservoir supply of fluid.  This part is shown in longitudinal section; the steel plunger, I, is firmly secured to wheel, F, the long hub, H, of which is provided with a screw thread on its inner side, which thread screws upon the exterior of pump barrel, K. After first filling the interior of the pump barrel with fluid, the said hub is screwed upon the pump barrel, causing the plunger, I, to force the fluid into the fluid chamber and passage way leading to the dial gauge, causing the hand or pointer to move to any predetermined pressure on dial, in advance of pressure applied in the high pressure chamber at D. The purpose accomplished in this act is to give the least possible movement of the pointer to record any maximum pressure, as, for example, assuming that 20,000 lb. was the expected pressure from any one explosive, then the pointer, by the means above described, can be set at, say, 18,000 lb., in which event the pointer is reduced to the minimum movement of only 2,000 lb. to register 20,000 lb.

It will be evident that much greater accuracy of measurement of maximum pressures can be obtained by the minimum movement of the pointer, as both the inertia and the momentum are reduced to the minimum quantity.  The subsidence of pressure resulting from explosives being about as sudden as the creation of pressure, causes the pointer to move too rapidly for correct ocular observation, on which account a static electric current is employed, causing a stream of electric sparks to shoot off from the end of the pointer, B, to the brass outer ring, M. The gauge is insulated for that purpose by glass plate, S, which is secured concentrically to the gauge proper and the ring, M. Binding posts for the electric wires are provided