[65] The length of the line was 52 miles, but as it was considered that auxiliary stationary power would in any case be necessary on the 10½ miles of very steep inclines, the cost of the Atmospheric apparatus is taken on 41½ miles.
[66] It must be remembered that beyond the South Devon Railway was the projected railway through Cornwall, which, with its long and heavy gradients, was, in all its features, even more suitable than the South Devon for the application of the Atmospheric System. Had that system succeeded, and been introduced on the Cornwall Railway, a very great saving might have been made in the cost of the works of this line.
[67] I.e. before the Croydon and Epsom Committee. See above, p. 138.
[68] It will be found on pp. 35-52 of the Minutes of Evidence taken before the Atmospheric Committee (ordered to be printed April 24, 1845).
[69] This appeared with sufficient clearness from the general comparison between vacuum, weight of train, and speed. The exact appropriation of the force employed was shown by some dynamometric experiments made on the line.
The highest speed recorded was 68 miles per hour, with a train of 28 tons, the speed averaging 64 miles per hour for four level miles of the line, the vacuum being 16 inches. This speed should have exhibited a resistance of about 21 lbs. per ton, or 588 lbs., as the running resistance or friction, and 645 lbs. for the resistance of the air; in all 1,233 lbs. Now, the pressure due to 16 inches of vacuum on the piston is 1,390 lbs., which gives 157 lbs. as the friction of the piston; a result which corresponds sufficiently well with a direct dynamometric experiment.
Going to the other extreme, there are numerous records of trains of 100 tons which attained, on a level of four miles in length, average speeds of from 30 to 35 miles per hour, with 16·5 inches of vacuum, one train of 103 tons going 32·4 miles per hour with 16·9 inches of vacuum.
[70] This valve consisted of a number of long delicate blades of spring steel, arranged parallel to each other, as in a musical box, but with wider intervals. These plates rested on a series of truly faced bars, which crossed the end of the air-passage. The slightest pressure outwards lifted the springs; and as the area of opening was large, a very free passage was given to the air. On the current ceasing, the blades instantly, yet without shock, replaced themselves in contact with the bars, clipping them tightly under a very small reverse pressure, and effectually closing the passage. Their merit consisted in their being almost without weight, and thus promptly re-closing the aperture by a delicate elastic reaction.
[71] Trains frequently arrived late on the Atmospheric portion of the South Devon Railway, owing to its being at the end of the long trunk line from London to Exeter, and having at its other end a locomotive line contending with very heavy gradients.
[72] It may be mentioned that, from the date of the abandonment of the Atmospheric System, he refused to receive any remuneration for his professional services as engineer beyond a nominal retaining fee.