The works being carried on at Bardonnêche are on a larger scale than at Modane; so we will, with our readers' permission, suppose ourselves arrived in company at the former place, and the first point which we will visit together will be the large house containing the air-compressing machinery. Before entering, however, we will throw a glance at the exterior of the building. We find before us, as it were, _two_ houses, in a direct line one with the other--one situated at the foot of a steep ascent; and the other at about seventy or eighty feet above it, on the side of the mountain. These two houses are, however, but _one_, being joined by ten rows of inclined arch-work. Along the summit of each row of arches is a large iron pipe, more than a foot in diameter. These ten pipes, inclined at an angle of about forty-five degrees, come out of the side of the upper house, and enter the side of the lower house, and serve to conduct the water from the large reservoir above to the air-compressing machinery, which is arranged in the house below, exerting in this machinery the pressure of a column of water eighty-four feet six inches in height. On entering the compression-room, we have before us ten compressing-machines, precisely the same in all their parts--five on the right hand, and five on the left, forming, as it were, two groups of five each. In the centre of these two groups are two machines, in every respect like a couple of small steam-engines, only they are worked by compressed air instead of steam, and which we will call _aereomotori_. Each of these aereomotori imparts a rotary motion to a horizontal axis extending along the whole length of the room, and on which are a series of cams, which regulate the movements of the valves of the great compressors. This axis we will call the "main shaft." One group of five compressors is totally independent of the other, and has its aereomotore with its main shaft; but still, with one single aereomotore, by means of a simple connecting apparatus, it is possible to work one or the other group separately, or both together; also, any number of the ten compressors can be disconnected for repairs without affecting the action of the rest, or may be injured without conveying any injury to the others. In front of each of the ten compressors are placed cylindrical recipients, in every respect like large steam-boilers, except that they have no fire-grate or flues, each having a capacity of seventeen cubic metres, or five hundred and eighty-three cubic feet. These recipients are put into communication one with the other by means of a tube similar to a steam-pipe connecting a series of steam-boilers; and each connection is furnished with a stop-valve, so that any one recipient can be isolated from the rest.