The experience acquired by our engineers tells us that the very maximum gradient an engine on the ordinary principle can climb up is 1 in 25, or 211 feet in a mile, but this can only be for a few yards, and with what is known, in locomotive language, as “a good run at it.” On the Sœmmering, as has already been stated, the average gradient on the north side is 1 in 47, or 111 feet in a mile; on the south side 1 in 50, or 105 feet in the mile; yet these gradients, favourable as they are, compared with the gradient of an ordinary roadway over a high mountain pass, could only be obtained at a cost of £98,000 a mile. Equally costly was the Giovi incline, constructed to surmount the Apennines near to Genoa. The worst gradient on it for a short distance is 1 in 29, or 160 feet in a mile; its best 1 in 50, or 106 feet in a mile. The ruling gradient of the beautiful railway over the Apennines between Pistoja and Bologna is 1 in 40, or 132 feet to the mile. The distance from Pistoja at the foot of the mountain on the east to Poretta at the foot of the mountain on the west, by the old road, is 25 “_chiliometres_” (kilometres), a little more than 15½ English miles, but in order to obtain for the railway the average gradient of 1 in 40 we have just mentioned, it has been necessary to extend the distance from 25 chiliometres to 40, equal to 25 English miles. Each of these 40 chiliometres has cost 1,000,000 francs, or £40,000, making the total cost of the railway independent of the special rolling stock for working it, £1,600,000, or at the rate of £64,000 a mile. If at the time that Mr. Fell was engaged in connection with the construction of this very fine work, he had had his ideas matured respecting the centre-rail system, and that consent had been given to the line being laid down in accordance with it, the distance to traverse would have only been 15½ English miles; many expensive works would have been avoided, and the total cost of the line would not have exceeded £400,000—probably it would have been considerably less. Another matter worthy of consideration is, that this Apennine Railway consumed eleven years in its construction. Not very protracted, considering that there are nearly 7 miles of tunnels (39 in number), of which 23 are on the ascent from Pistoja, and 16 on the descent to Poretta. The longest is 1⅞ mile, the second longest 1⅝ mile, the length of the others is pretty nearly equal. Nearly one-eighth of what is not tunnel is bridge or viaduct; some of the latter across ravines 300 to 400 feet long, and nearly 180 feet high. They are, in fact, double viaducts, one built on the top of the other, and reminding one of the two tiers of guns of an old-fashioned two-decker. According to the testimony of M. Desbriere, Mr. Fell would have accomplished an equally efficient railway on or through the pass in a couple of years. He would have had no tunnels, unless when, occasionally, he might want to get by a short cut through a projecting ledge of rock or mountain instead of round it, and, as to bridges and viaducts, a twentieth part of those now existing would have amply sufficed him. Before quitting this subject, we would observe that Captain Tyler sets down the difference between the length of a mountain road with gradients of 1 in from 12 to 15 as one-half that of a roadway with gradients of 1 in 40. In practice, however, it will probably be found that the difference will hardly be so great—but there can be no doubt of its not being less than as 3 to 5.