The air-locks A A, heretofore as a rule placed above the surface of the water, are located within the roof of the airchamber, and access is had to them through brick wells F, G, thus avoiding the inconvenience and delay of adding new joints under the locks.

Fig. 61.

The sand-pumps E are placed on the roof of the chamber, their suction pipes extending through the chamber to the sand. The action of these pumps is very simple. A stream of water is forced down the pipe B, (Fig. 61), and discharged near the sand into the pipe A, through the annular jet C. The jet creates a vacuum below it, by which the sand is drawn into the second pipe, the lower end of which is in the sand, and the force of the jet carries it up to the mouth of the pump so soon as it

B

C.

passes

The abutments at the east end of the bridge (Figs. 61 a and 61 b) differed in some of the details of their construction from the piers.

E

Fig. 61, represents a vertical seo

tion of a sand-pump.

A, pump barrel.

B, injection pipe.

C, annular jet.

Fig. 61 a, is a part plan and part section of the east abutment of the St. Louis Bridge. Fig. 61 b, is a vertical section of the same.

I, is the main shaft.
KK, the side shafts.
MM, iron girders.
00, the air-locks.
PP, the air-chamber.
QQ, the timber girders.
RR, the timber deck.
SS, the iron sheeting.

TT, the timber sides of the caisson.

Fig. 61 b.

. The main shaft had two air-locks at the lower end, each 8 feet in diameter, having about four times the capacity of the one used in the piers. There were also two other shafts and air-locks which were introduced to secure additional safety. This caisson was probably sunk to a greater depth than any other in the world by the pneumatic process.

It was sunk to the native rock, which was 136 feet below high-water mark, and 94 below the extreme low-water mark. It was about 110 feet below the surface of the water at the time it was completed. It was extremely hazardous to the health and even lives of workmen to be kept under the pressure of over three atmospheres for a long time. The greatest security was found in changing them every three or four hours.

Candles burned very readily at this depth and pressure. After a depth of about 80 feet was reached, the candles were inclosed in a strong glass globe, the inside of which communicated with one of the shafts, and the pressure was regulated by a small tube passing through the globe and containing a check valve. In this way the candles burned in an atmosphere whose pressure was about the same as the external air. (See London Engineering, 1870 and 1871.)

East River Bridge. The caisson for this bridge is composed almost wholly of wood. The air-chamber (Fig. 62) is nine feet six inches high, the roof being made of fifteen courses of timbers, one foot thick, the lower five (A) being

CAISSON OF EAST RIVER BRIDGE.

235

laid solid, the upper ten (C) crossing in alternate layers, and placed about a foot apart, the spaces between the timbers being filled with concrete. The sides (B) of the air-chamber are V shape, made very solid, nine feet thick at top, and eight inches at the bottom, which is heavily shod with iron. Between

Fig. 62 represents a vertical section of the Brooklyn Caisson of the East River Bridge.

A, lower timber courses of roof, laid solid.

B, timber sides of air-chamber.

C, upper timber courses of roof, laid crosswise, and spaces filled in with concrete.

D, masonry of pier.

E, dam to prevent water from reaching shafts.

F, air-shaft and lock.

G, supply shaft,

H, excavation shaft.

I, heavy timber partitions.

K, air-chamber.

the fourth and fifth courses of the roof is laid a sheet of tin, which is continued down underneath the outside sheathing. The air-chamber is divided into six compartments by heavy timber girders. The shafts through which the heavy material is raised extend below the level of the excavation at the bottom, and are constantly open; but the compressed air is prevented from escaping by a column of water, which is maintained at nearly the same height as the water in the river by the pressure of the compressed air. If the pressure of the air should be made to greatly exceed that at which it is ordinarily maintained, it would blow all the water out of the shaft

and the air would entirely escape, but every necessary precaution was used to keep a proper pressure of the air. An accident of this kind once took place in the Brooklyn caisson.

VII.

CONSTRUCTION OF MASONRY.

460. Under this head will be comprised whatever relates to the manner of determining the forms and dimensions of the most important elementary components of structures of masonry, together with the practical details of their construc

tion.

461. Foundation Courses. As the object of the foundations is to give greater stability to the structure by diffusing its weight over a broad surface, their breadth, or spread, should be proportioned both to the weight of the structure and to the resistance offered by the subsoil. In a perfectly unyielding soil, like hard rock, there will be no increase of stability by augmenting the base of the structure beyond what is strictly necessary for stability in a lateral direction; whereas in a very compressible soil, like soft mud, it would be necessary to make the base of the foundation very broad, so that by diffusing the weight over a great surface, the subsoil may offer sufficient resistance, and any unequal settling

be obviated.

462. The thickness of the foundation course will depend on the spread; the base is made broader than the top for motives of economy. This diminution of the volume (Fig. 63)

is made either in steps, termed offsets, or else by giving a uniform batter from the base to the top.

When the foundation has to resist only a vertical pressure,

an equal batter is given to it on each side; but if it has to resist also a lateral effort, the spread should be greater on the side opposed to this effort, in order to resist its tendency, which would be to cause a yielding on that side.

463. The bottom course of the foundations is usually formed of the largest sized blocks, roughly dressed off with the hammer; but if the bed is compressible, or the surfaces of the blocks are winding, it is preferable to use blocks of a small size for the bottom course; because these small blocks can be firmly settled, by means of a heavy beetle, into close contact with the bed, which cannot be done with large-sized blocks, particularly if their under surface is not perfectly plane. The next course above the bottom one should be of large blocks, to bind in a firm manner the smaller blocks of the bottom course, and to diffuse the weight more uniformly over them.

464. When a foundation for a structure rests on isolated supports, like the pillars, or columns of an edifice, an inverted or counter-arch, (Fig. 64,) should connect the top course of the foundation under the base of each isolated support, so that the pressure on any two adjacent ones may be distributed over the bed of the foundation in the interval between them. This precaution is obviously necessary in compressible soils.

The reversed arch is also used to give greater breadth to the foundations of a wall with counterforts, and in cases where an upward pressure from water, or a semi-fluid soil requires to be counteracted. In the former case the reversed arches are turned under the counterforts; in the latter they form the points of support of the walls of the structure.

465. The angles of the foundations should be formed of the most massive blocks. The courses should be carried up uniformly throughout the foundation, to prevent unequal settling in the mass.

The stones of the top course of the foundation should be