the materials were thrown, and they were discharged thoroughly mixed and ready for use into wheelbarrows at the lower end. It is stated that this simple machine delivered from 280 to 350 cubic feet in ten hours.

The concrete is usually thrown down into the pile from the bell or lock. At the bridge at Szegedin the double locks were, alternately, nearly filled with the concrete, and it was raked out from them and thrown into the pile; care being taken to work it in well by hand, around the flanges and joints.

Bridge over the Savannah River on the line of the Charleston and Savannah Rail Road. The air-locks on these piles were similar to the Harlem plan. Light was admitted into the air-lock by means of large bulls-eye glasses, and thence into the body of the pile in the same way, but this mode was found to be worthless, on account of the mud in the bottom of the air-lock which covered the glass. The engineer employed a secondary small air lock so that the material which was brought into the main one could be discharged at any time, and thus the work go on with less interruption, and the bulls-eyes became more serviceable. With the secondary air-lock the work progressed more rapidly; the ratio for a given amount of work being

By a fortunate discovery the engineer discovered that the pressure of the air in the pile was sufficient to force sand from

the bottom of the pile through a vertical pipe to a height above the surface of the water outside the works. A sort of telescopic tube was attached to the lower end of a pipe so that it could be easily moved downward as the excavation progressed. This greatly facilitated the progress of the work, for it was found that to do a given amount of work the ratio

was

Time by old air-lock...

14

Time by blowing out sand ==28

This mode of excavation has been adopted to some extent in the caissons of the East River Bridge. This process also secures thorough ventilation. The same plan has also been used in the Omaha Bridge and Leavenworth Bridge with equally good results.

It is Sometimes very difficult to keep the tubes vertical. When they begin to incline efforts should be made immediately to bring them to an erect position.

In some cases

wedges or blocks placed under the lower side and suddenly relieving the pressure will correct the evil. An ingenious mode

bored several holes through the tubes on the upper side, through which the compressed air escaped and thus disturbed the soil and relieved the pressure on that side so that it would sink faster. Strong levers have been used to pull on the top whilst the tube was sinking, but not with very marked reIn at least one very obstinate case, in which the holes did not alone effect the desired result, a ram was used in upper side, combined with the action of a strong lever, combination with the other devices and the erect position was quickly secured. The jar produced by the ram whilst the

sults.

on the

tube

was sinking seemed to give great effect to the other

devices.

Gen. W. S. Smith, who had charge of the construction of the foundations of the Omaha and Leavenworth Bridges, is of opinion that a pneumatic caisson, surmounted by masonry, is cheaper and better than pneumatic pile piers, but it is evident that circumstances may often determine which is preferable in any particular case.

459, Pneumatic Caissons. The application of compressed

I of the railroad bridges recently constructed in Europe; by air for laying foundations has been further extended in some

as an

using wrought-iron caissons of sufficient dimensions to serve envelope, or jacket, for the masonry of an entire pier; gradually sinking the whole to the requisite depth, by excavating the soil within the pier to the desired level.

and

The caissons (Figs. 57, 58) for this purpose were divided into two compartments.

The lower A (Fig. 57), which served as a chamber for the workmen, for excavating the soil, was strongly roofed at top, with iron bars and iron sheeting, to bear the weight of the masonry that rested upon it; and was securely buttressed on the sides to resist the inward pressure of the soil on the outside. The upper chamber, B, served as an ordinary caisson, fitting closely to the masonry on the sides, and rising sufficiently above it to exclude the water during the construction of the masonry: the body of which, composed of béton with a facing of stone, was gradually raised as the caisson was sunk through the earth overlying a bed of rock upon which the pier was finally to rest.

The working chamber A was connected with two bells C, C, by two vertical iron cylinders D, D (Fig. 57), for each bell; these cylinders serving as a communication between the working-chamber and bells, for the passage of the workmen from one to the other, for raising the excavated soil, and as a passage for the compressed air forced in by the air-pumps.

Each bell contained two air-locks for communicating between it and the exterior; and a hoisting gearing for the excavated soil; the filled buckets ascending through one cylinder, and the empty ones descending through the other.

The lower chamber, the bottom of which was open, was kept filled with compressed air of sufficient density to exclude the water, and enable the workmen to excavate the soil.

The caisson was gradually sunk, by the weight of the superincumbent mass, as the soil below was removed.

So soon as the rock-bed was reached, the surface was thoroughly cleaned off, and levelled under the edges of the bottom of the caisson, and the chamber A was gradually filled in with masonry closely up to its roof. Finally the eylinders D were removed, and the wells occupied by them in the body of the pier, filled with béton.

As a matter of interest, on the subject of laying foundations by means of pneumatic piles and caissons, a few additional facts in connection with the examples above cited will not be out of place here.

Bridge over the Scorff. In the example of the bridge at L'Orient over the Scorff, the river-bed is a stratum of mud, forty-six feet in depth, resting upon a surface of hard schistoze rock more or less inclined and uneven. The level of mean tide is about sixty feet above the rock surface; that of the highest tide seventy feet.

The caissons used in this example were designed for the piers of a stone bridge, and were about forty feet long and twelve feet broad. The bells, or upper working chambers, were ten feet high and eight feet in diameter; the lower working-chamber ten feet high; and the cylinders, for communication between them, two feet and a half in diameter.

The caissons were built of sheet-iron, in zones decreasing in thickness from the top to the bottom; but not having been buttressed within against the pressure of the water, as the lower working-chamber was, they yielded and got out of shape.

In a subsequent structure of nearly the same dimensions, for a railroad bridge at Nantes, the same failure took place, and precautions were then taken against it by the insertion of cross-stays, which were removed as the masonry was carried up. In the caissons used in this case, the bells and air-locks were made larger. Each air-lock had three separate compartments; one for the passage of the workmen which could contain four men; one for the barrows by which the excavated soil was removed, and one for the concrete to fill up the lower working chamber, when the excavation was completed.

St. Louis Bridge. The caissons for the two piers of this bridge differ in no material respect, so that a description of one will equally apply to the other. The air-chambers are nine feet high, the sides being formed of 4-inch plate iron in the larger, and -inch in the smaller. The air-chamber is simply a huge diving-bell of the full size of the pier. The iron plates K, K (Fig. 59), forming its roof, are of -inch thickness. Transversely over this and riveted firmly to it are thirteen iron girders L, at intervals of five and a half feet. Beneath the roof two massive timber girders C, C (Figs. 59 and 60), in an opposite direction to the iron ones, divide the air-chamber into three nearly equal parts. Communication between the three divisions is had through openings made for this purpose in the girders. These timber girders are intended to rest on the sand and support the roof from below. The sides of the air-chambers are strongly braced with plate iron brackets O O, stiffened with angle iron. Between the brackets is placed all around the chamber a course of strong timbers, the bottom of which is level with that of the girders, intended to rest on the sand and assist in supporting the weight. The support given by the timbers, together with the buoyant power of the compressed air in the chamber and the friction of the sand on the sides, is the only means relied on to sustain the pier in its gradual descent to the rock.

Fig. 59-Represents the plan of the caisson of the East pier of the Illinois and St. Louis Bridge, Fig. 60 represents transverse section of the same. A. air-locks. B, air-chamber. C, timber girders. D, discharge of sand. E, sand-pumps. F, main entrance shaft. G, side shafts. H, iron sides. I, bracing for H. K, iron deck or roof. L, iron girders. O, strengthening girders.