The operation of grinding was formerly regarded as a very dangerous one, from the liability of the stones to burst in consequence of their enormous weight and the velocity with which they revolve; but, about twenty years since, a new method of clamping the stone was adopted, by means of which the danger of bursting is much diminished.  The last explosion which took place in this department occurred about nine years ago.  The operation of grinding, however, is objectionable also from the very unhealthy nature of the work.  Immense quantities of fine dust fill the air, and the premises are always drenched with water, making the atmosphere damp and unwholesome.

In former times, it was customary to grind bayonets as well as barrels; but the former are now milled instead, thus making an important saving in expense, as well as gain in the health of the establishment.  No mode, however, has yet been devised for dispensing with the operation of grinding the barrel; but the injury to the health, in this case, is much less than in the other.

When the barrels are nearly finished, they are proved by an actual test with powder and ball.  To this purpose a building at the water-shops, called the proving-house, is specially devoted.  It is very strongly built, being wholly constructed of timber, in order to enable it to resist the force of the explosion within, and contains openings in the roof and at the eaves for the escape of the smoke, a very large number of barrels being proved at once.

The barrels are subjected to two provings.  In the first, they are loaded with a double charge of powder and two balls, thus subjecting them to a far greater strain than they can ever be exposed to in actual service.  In the second proving, only the ordinary charge is used.

The interior of the proving-house is very happily arranged for the purpose to which it is put.  On the right-hand end of the building as you enter, and extending across it, is a platform of cast-iron, containing grooves in which the muskets are placed when loaded.  A train of gunpowder is then laid on the back side of this platform, connecting with each barrel, and passing out through a hole in the side of the building near the door.  A bank of clay is piled up on the opposite side of the room, into which the balls are thrown.  Only one fatal accident has occurred at the armory during the last two years, and this occurred in the proving-house.  When the muskets are brought in, they are placed upright in frames, which, when full, are laid down upon the platform.  Five barrels are placed in a frame, and these five exploded while the man was putting them in the proper position for laying them down, and ten balls were plunged into him.  No satisfactory explanation could ever be obtained of the cause of the premature explosion.

About one per cent. of the barrels burst under this trial, although under the old process of welding there was a loss of nearly two per cent., or one in sixty.