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Summary of the average Items of the Construction of a Mile of Railway. By Mr. DEMPSEY.

"The average quantities, per mile, of the several items which are involved in the formation of a double line of railway, of the 4 ft. 8 in. gauge, up to the completion of the permanent way, and exclusive of the stations and buildings, and locomotive and carrying stock, may be computed as follows:

"The quantity of excavations in 342 miles of double line of railway, (comprised in ten railways,) amounted to 35,338,000 cubic yards, giving an average of about 103,330 yards per mile, or 58.71 cubic yards of earth-work for each yard forward of the line. Assuming the width of the formation level to be 10 yards, or 30 feet, (which is about the average,) with an additional width of 5 yards on each side, for ditches, hedges, &c., the slopes at 11⁄2 base to 1 of height,—and also assuming the whole line to be, either in cutting or embankment, of an average depth of height of 11 feet, we shall require 56.73 cubic yards of earth-work per yard forward of the line. This is sufficiently near to the actual average of 58.71 yards to answer the pur

pose of this general calculation. The average width

of land required will thus be,

Central width. Base of Slopes.

Ditches, &c.

30+ 16.5 + 16.5 + 15 + 15 = 93 ft. or 31 yds., which will give about 11 acres of land per mile. Allowing for severance, &c., this may be assumed at 12 acres.

"The quantity of ballasting, 30 feet wide, and 18 inches thick, will equal 5 cubic yards per yard forward, or 8800 cubic yards per mile.

"The sleepers, transverse, 8 feet long, and 10 by 5 inches, placed 2 feet 6 inches apart, will require 11.733 cubic feet, or 235 loads of timber; or 4224 sleepers per mile.

"The chairs required, supposing the rails to be rolled in lengths of 15 feet each, will be 1408 joint chairs, and 7040 intermediate; and their weight, reckoning each joint chair at 20 lbs., and each intermediate chair at 15 lbs., will be 12 tons 11 cwt. 1 qr. 20 lbs., and 47 tons 2 cwt. 3 qrs. 12 lb., respectively, or 59 tons 14 cwt. 1 qr. 4 lbs. altogether.

"The rails, assuming the weight at 56 lbs. per yard, will weigh 176 tons,-1408 lengths being required.

"If two oak trenails and two iron spikes be required for each chair, 16,896 of each will be wanted per mile, with 8448 wooden keys for fixing the rails in the chairs.

"If felt be interposed between the chairs and sleepers, and the former be assumed at 10 x 5 inches bearing surface, 2933 square feet of felt will be required per mile.

"The timber in the side fences, formed of posts 8 feet long, 6 × 4 inches, 9 feet apart, with four rails 5 × 2 inches, and intermediate upright stay 3 × 2 inches, will consume as follows: 1174 posts = 1565 cubic feet; 4696 rails = 3666 cubic feet; 1174 stays 269 cubic feet; or a total of 110 loads.

"Of the masonry, timber, iron, &c., &c., in bridges, viaducts, culverts, drains, retaining walls, &c., scarcely any estimate can be formed. Taking the average of a few cases, the masonry would appear to amount to about 110,000 cubic feet per mile; but in some cases from 30 to 50 per cent. of this quantity is substituted by timber and iron."

Weight of Rails.

On railways with much heavy traffic, the weight of the rails should be, to insure firmness and durability, as on the London and North-western Railway, about 75 lbs. per yard, and their bearing-surface about 21⁄2 inches broad. The best distances for the bearings being about 2 feet 9 inches to 3 feet asunder.

Atmospheric Railway.

An experimental line of Atmospheric Railway on Hallette's principle, about 100 yards long, has been laid down, to exhibit its peculiar valve, and its working power, on a small scale. The valve is closed by longitudinal caoutchouc pipes, covered with cotton and leather, and filled with compressed air, at about 5 lbs. to the inch pressure. The wear and tear of these elastic lips, by the continual rubbing of the wedge which opens them as the train passes, can be satisfactorily ascertained only by experiments on a large practical scale.

Menai Tubular Bridge.

The tubular bridge designed by Mr. Stephenson for crossing the Menai Straits, or the line of the Chester and Holyhead Railway, is proposed to be rectangular, and of the following dimensions, viz. length 450 feet, width 15 feet, height 30 feet; made of iron plates one inch in thickness. Numerous experiments have been made on the strength of iron tubes, by Messrs. Hodgkinson and Fairbairn, to determine the requisite strength, and the weight it would support. The estimated strength of this tube would be equal to 1100 tons applied in the centre, including its own weight; or 747 tons, deducting its own weight. But this being the full strain that the tube would bear without breaking, a

much less weight must be fixed upon as within the point of safety. The addition of chains is proposed to add to the support of the tube, and experiments are still in progress to determine a form that would sustain a more considerable weight. Its practicability has been established, in the opinion of the engineers, by the results of experiments on a tube 75 feet long, 21 feet wide, and 4 feet deep, weighing about 5 tons; which broke with a weight on the middle of 35 tons.

Resistance to Railway Trains.

The resistance of the air to railway trains is estimated, by Mr. Barlow, at not more than ten pounds on each ton weight, on the average. The loss of velocity estimated by comparison of the actual with the theoretical velocity, is caused by the consumption of power in overcoming the inertia of the train, and not from defect or loss of power in the action of the engine.

A paper was read at the meeting of the British Association in September, by Mr. Scott Russell, on "The law which governs. the resistance to the motion of railway trains at high velocities." His experiments have been undertaken "on a large scale, with railway trains of a great variety of size and weight, and at velocities as high as sixty-one miles an hour," and were combined with those formerly

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