The emery which is used in these polishing operations, as well as for a great many similar purposes in the arts, is obtained by pulverizing an exceedingly hard mineral that is found in several of the islands of the Grecian Archipelago, in the Mediterranean. In its native state it appears in the form of shapeless masses, of a blackish or bluish gray color, and it is prepared for use by being pulverized in iron mortars. When pulverized it is washed and sorted into five or six different degrees of fineness, according to the work for which it is wanted. It is used by lapidaries for cutting and polishing stones, by cutlers for iron and steel instruments, and by opticians for grinding lenses. It is ordinarily used in the manner above described, by being applied to the circumference of a leathern covered wheel, by means of oil or of glue. Ladies use bags filled with it, for brightening their needles. Emery is procured in Spain, and also in Great Britain, as well as in the Islands of the Mediterranean. PROVING. [Illustration: THE PROVING HOUSE.] When the barrels are brought pretty nearly to their finished condition, they are to be _proved_, that is to be subjected to the test of actual trial with gunpowder. For this proving they are taken to a very strong building that is constructed for the purpose, and which stands behind the Stocking Shop. Its place is on the right in the general view of the Armory buildings, and near the foreground--though that view does not extend far enough in that direction to bring it in. The exterior appearance of this building is represented in the above engraving. It is made very strong, being constructed wholly of timber, in order to enable it to resist the force of the explosions within. There are spacious openings in lattice work, in the roof and under the eaves of the building, to allow of the escape of the smoke with which it is filled at each discharge; for it is customary to prove a large number of barrels at a time. The barrels are loaded with a very heavy charge, so as to subject them to much greater strain than they can ever be exposed to in actual service. The building on the left, in the engraving, is used for loading the barrels, and for cleaning and drying them after they are proved. The shed attached to the main building, on the right hand, contains a bank of clay, placed there to receive the bullets, with which the barrels are charged. The arrangement of the interior of this building, as well as the manner in which the proving is performed, will be very clearly understood by reference to the engraving below. [Illustration: INTERIOR OF THE PROVING HOUSE.]