EPROUVETTE is a machine for proving or testing the strength of gunpowder. It was invented or suggested in the last century by Robins, but was greatly improved by Dr. Hutton.
The gun E. determines the strength of gunpowder by the amount of recoil produced. A small gun, usually a " half-pounder," is fixed to the lower end of an iron rod; its base being adjusted to an arm projecting from the rod; or else it is suspended from an iron frame. A horizontal steel axis is fixed to the rod or frame about which the gun may vibrate. A pointed iron rod or style projects downwards from the lower side of the gun, and touches a groove filled with soft wax; the groove is so shaped that, when the gun recoils, the point cuts a path for itself along this wax; and the length of this path determines the amount of recoil. Sometimes a brass graduated arc with an index is used instead of the pointed style and the waxed groove; but the principle of action is just the same. On the arc the recoil should vary from 26° for new fine-grain powder to 20° 5' for old powder of coarse grain. This system of proof is resorted to annually at minor and foreign stations for the proof of all powder in store, to ascertain the amount of deterioration; five rounds constitute the minimum proof. Before the E. is
resorted to, the powder must pass the test of specific gravity, by weighing not less than 55 lbs. to the cubic foot.
The mortar E. determines the strength of gunpowder by the distance to which a ball is projected, instead of the distance to which the piece recoils. It is generally a mortar of 8-inch bore, in which 2 to 4 ozs. of powder is employed to propel an accurately turned iron shot to a distance'of about 120 yards. Other things being equal, the strong est gunpowder sends the shot to the greatest distance; and this is the usual mode adopted in testing gunpowder supplied to the government by various contractors.
The ordinary E. is an instrument shaped like a small pistol without a barrel, and having its breach chamber closed by a flat plate connected with a strong spring. On the explosion of the powder against the plate, it is driven back to a distance indexed according to the strength of the powder, and is retained at its extreme state of propul sion by a ratchet wheel.