Susa, 10⅓ kilometres (6½ miles), it is not much better, being 1 in 15, or 350 feet in the mile. There is a difference in the comparative elevations above the sea, of St. Michel and Susa of 715 feet, Susa being the lower of the two. Consequently, the Italian side of the pass is more precipitous than the French, for, whereas the total rise from St. Michel to the summit (a distance of 31 miles) is 4,165 feet, the descent from the summit to Susa (a distance of 18 miles) is 4,880 feet; in fact, the descent of the mountain only terminates as the town is entered. Of course, what are ascent and descent when going from France to Italy are reversed when coming in the opposite direction. Mr. John Fell,[118] a gentleman about whom we have now to speak, has been occupied during many years of his life in the construction of railways and other works in Italy, being associated in several of them with Mr. Thomas Brassey, the eminent contractor, and other English gentlemen, among whom may be mentioned Mr. William Jackson, now M.P. for North Derbyshire. Connected with the laying out and subsequent construction of the magnificent railway between Pistoja and Bologna, by which the Apennines are traversed and overcome by the locomotive, Mr. Fell’s attention was directed to the mighty cost of construction of this railway, as well as of that across the Sœmmering and of the Giovi Incline, by means of which last-named, Genoa obtains railway connection with all parts of the Italian system. Reasoning and reflecting, Mr. Fell’s mind was, by a most happy incidence, brought to bear upon the effect that would be developed if the adhesion of the locomotive were increased by the application of horizontal wheels to a third or intermediate rail. But, in order to understand and to appreciate the full force of this conception, it is necessary to explain that the power or motive force of a locomotive is derived from two sources—the amount of steam evaporated and the amount of adhesion, or, as it is familiarly called, “bite,” that the wheels have on the rails. If a railway were completely level, it would only be necessary, in the construction of engines for working it, to make them with as little weight of material as possible, consistent with the strength and solidity required for their complete efficiency. But as a railway on, or very nearly on, the level, does not exist, it is necessary, in building engines, to give them such weight as, in addition to their steam power, will ensure them efficient adhesion upon the railway. On railways with favourable gradients the additional weight of engine is trifling; but exactly in proportion as gradients become unfavourable, so has it been found necessary to increase the weight of engines. Mr. Fell’s system increases adhesion without increasing the weight of the engines,[119] except to the very trifling extent of that appertaining to the four horizontal wheels, and the machinery connected with them.