Such is the general arrangement of the Arsenal buildings, "on the hill." But it is only the lighter work that is done here. The heavy operations, such as rolling, welding, grinding, &c., are all performed by water-power. The stream which the Ordnance Department of the United States has pressed into its service to do this work, is a rivulet that meanders through a winding and romantic valley, about half a mile south of the town. On this stream are three falls, situated at a distance perhaps of half a mile from each other. At each of these falls there is a dam, a bridge, and a group of shops. They are called respectively the Upper, Middle, and Lower Water Shops. The valley in which these establishments are situated is extremely verdant and beautiful. The banks of the stream are adorned sometimes with green, grassy slopes, and sometimes with masses of shrubbery and foliage, descending to the water. The road winds gracefully from one point of view to another, opening at every turn some new and attractive prospect. The shops and all the hydraulic works are very neatly and very substantially constructed, and are kept in the most perfect order: so that the scene, as it presents itself to the party of visitors, as they ride slowly up or down the road in their carriage, or saunter along upon the banks of the stream on foot, forms a very attractive picture. THE MUSKET BARREL. The fundamental, and altogether the most important operation in the manufacture of the musket, is the formation of the barrel; for it is obvious, that on the strength and perfection of the barrel, the whole value and efficiency of the weapon when completed depends. One would suppose, that the fabrication of so simple a thing as a plain and smooth hollow tube of iron, would be a very easy process; but the fact is, that so numerous are the obstacles and difficulties that are in the way, and so various are the faults, latent and open, into which the workman may allow his work to run, that the forming of the barrel is not only the most important, but by far the most difficult of the operations at the Armory--one which requires the most constant vigilance and attention on the part of the workman, during the process of fabrication, and the application of multiplied tests to prove the accuracy and correctness of the work at every step of the progress of it, from beginning to end.