Second proof.—A sample of 1lb. from each lot, carefully weighed up, and a similar sample of the comparison powder, is exposed for three weeks in a box perforated with holes (called a damp chest), to the action of the atmosphere. This box is placed under cover, so that it is sheltered from the wet, but that the moisture can get to it. If, at the end of this time, there is a greater proportion of difference in range between them than one-twentieth, it is rejected. The pounds are also very carefully weighed up again, to ascertain the comparative absorption of moisture. This is called the hygrometric test.
REMARKS ON THE PROOF OF POWDER BY THE EPROUVETTES.
By eprouvettes or pendulum.
By comparing the results of the proofs by the eprouvettes with those furnished by the cannon pendulum (vide [plate 1], fig. 2 and 3), it will appear that the eprouvettes are entirely useless as instruments for testing the relative projectile force of different kinds of powder, when employed in large charges in a cannon. Powders of little density, or of fine grain, which burn most rapidly, give the highest proof with the eprouvettes, whilst the reverse is nearly true with the cannon.
Real use of eprouvettes.
The only real use of these eprouvettes is to check and verify the uniformity of a current manufacture of powder, where a certain course of operations is intended to be regularly pursued, and where the strength, tested by means of any instrument, should therefore be uniform.
Best proof, by service charges.
The only reliable mode of proving the strength of Gunpowder is, to test it with service charges in the arms for which it is designed; for which purpose the balistic pendulums (vide [plate 3]), are perfectly adapted.
Best proof for small arms.