[Illustration:  FIG. 59.—­NOBLE’S PRESSURE GAUGE.]

Noble’s Pressure Gauge.—­The original explosive vessels used by Captain Sir A. Noble in his first experiments were practically exactly similar to those that he now employs, which consists of a steel barrel A (Fig. 59), open at both ends, which are closed by carefully fitted screw plugs, furnished with steel gas checks to prevent any escape past the screw.  The action of the gas checks is exactly the same as the leathers used in hydraulic presses.  The pressure of the gas acting on both sides of the annular space presses these sides firmly against the cylinder and against the plug, and so effectually prevents any escape.  In the firing plug F is a conical hole closed by a cone fitting with great exactness, which, when the vessel is prepared for firing, is covered with fine tissue paper to act as an insulator.  The two firing wires GG, one in the insulated cone, the other in the firing plug, are connected by a very fine platinum wire passing through a glass tube filled with meal powder.  The wire becomes red-hot when connection is made with a Leclanche battery, and the charge which has previously been inserted into the vessel is fired.  The crusher plug is fitted with a crusher gauge H for determining the pressure of the gases at the moment of explosion, and in addition there is frequently a second crusher gauge apparatus screwed into the cylinder.  When it is desired to allow the gases to escape for examination, the screw J is slightly withdrawn.  The gases then pass into the passage I, and can be led to suitable apparatus in which their volume can be measured, or in which they can be sealed for subsequent chemical analysis.

The greatest care must be exercised in carrying out experiments with this apparatus; it is particularly necessary to be sure that all the joints are perfectly tight before exploding the charge.  Should this not be the case, the gases upon their generation will cut their way out, or completely blow out the part improperly secured, in either case destroying the apparatus.  The effect produced upon the apparatus when the gas has escaped by cutting a passage for itself is very curious.  The surface of the metal where the escape occurred presents the appearance of having been washed away in a state of fusion by the rush of the highly heated products.

The Pressure Gauge.—­The pressure is found by the use of a little instrument known as the pressure gauge which consists of a small chamber formed of steel, inside of which is a copper cylinder, and the entrance being closed by a screw gland, in which a piston, having a definite sectional area, works.  There is a gas check E (Fig. 60) placed in the gland, and over the piston, which prevents the admission of gas to the chamber.  When it is desired to find the pressure in the chamber of a gun, one or more of these crushers are made up with or inserted at the extreme rear end of the cartridge, in order to avoid their being blown out of the gun when fired.  This, however, often takes place, in which case the gauges are usually found a few yards in front of the muzzle.  The copper cylinders which register the pressure are made 0.5 inch long from specially selected copper, the diameters being regulated to give a sectional area of either 1/12 or 1/24 square inch.