but the action is the reverse of ordinary usage. The piston (fig. 24) is kept stationary and in continuous contact with the pile head, while the blow is administered by the lower end of the heavy cast-iron cylinder, moving up and down under steam pressure. The movement of the cylinder is guided by rollers behind the main leaders, and the arrangement involves a sliding steam feed pipe (which is a special feature of the system), with a flexible rubber connection to the supply pipe from boiler. At the head of the cylinder is a two-way cock, regulated by a double-armed lever, which, when pulled down on one side, exhausts the cylinder, and on the other admits fresh steam. There is also a double-action machine, in which steam pressure is applied alternately to each side of the piston, thus increasing the force of the blow. The stroke is about 3 feet, and blows can follow one another with great rapidity. From observations of a Whitaker machine in single action with 80 to 90 lbs. steam pressure, the author finds that 35 blows per minute can be delivered at full stroke, or 60 blows per minute with a stroke of 12 inches. In double action 45 blows per minute were delivered with a stroke of 2 feet. The weight of the cylinder was 1 ton.

A similar machine, known as the Cram Pile Driver, manufactured in America, has a hammer fastened to the lower end of the cylinder, and is supplied with steam through a hollow piston-rod. The original Nasmyth hammer is also used, in which the hammer is attached to the piston, the cylinder remaining stationary and being confined between the upper ends of two vertical and parallel I or channel beams, the lower ends of which enclose a hollow, conical bonnet casting, fitting over the head of the pile. This casting is open at the top, and through it the blow is administered. When steam is admitted to the cylinder, the hammer is lifted about 30 to 40 inches and then allowed to fall, generally by the automatic opening of an escape valve.

Piling machines of the steam-hammer type consume from 1 to 2 tons of coal per day, working with a boiler pressure of 50 to 75 lbs. per square inch, and can deliver blows at the rate of about 60 per minute. They need three men in attendance.

The disadvantage attending them is the liability of the pilehead to crushing or brooming, which, combined with the escape of moisture from the cylinder, reduces it, if in the least degree soft or sappy, to a saponaceous condition. The effect of this is to materially diminish the force of the blow, as is evidenced by the following particulars of the driving of a green Norway pile by a Nasmyth steam hammer *

:

* Whittemore on "The Efficiency of Pile Driving," Min. Proc. Inst. C.E., vol. lxxvi., p. 399.

The total number of blows was 5,228. A similar pile, which was not adzed or sawn, required 9,923 blows to descend to the same depth. The ram weighed 2,800 lbs. and fell 3 feet sixty-five times per minute. The friction caused by the working of the fibres on each other, under the blows of the hammer, was sufficient to ignite and burn the interior of the head of the pile from side to side.

A third type of pile driver is the Electric Pile Driver, in which advantage is taken of the temporary magnetisation of wrought iron to make an electromagnet of that material attach itself by contact to the cast-iron monkey. The two parts are then lifted by the winch. On switching off the current the monkey falls, and the magnet is caused to follow it down ready for lifting again. The monkey is of the ordinary kind, with an upper planed surface. The magnet is connected by wires to the motor on the winch. The illustration (fig. 25) is of one manufactured by the New Southgate Engineering Co., Ltd.

Hydraulic Method. While piles readily respond to the motive force of the ram in ordinary ground, and even in stiff clay, their progress through sand and gravel is not so satisfactory, and the ordinary methods of driving have generally to be abandoned, either wholly or partially, in favour of the water jet. The principle of this method consists in transforming the sand immediately beneath the pile into quicksand, by saturating it with water under pressure, a condition which enables the pile to sink by its own weight or with very little assistance. The water is conducted to the foot of the pile by means of wrought iron gas piping having a short returned end, provided with a nozzle or pierced with holes, which passes underneath the pile. This last is not usually pointed, but left with a butt end, which favours perpendicularity in driving. The descent of the pile may be expedited by a static weight, or by the direct downward pull of a rope passing through sheaves to a winch. When the pile has been sunk to a sufficient depth, the nozzle of the water pipe is turned through a quadrant to clear the pile and brought up to the surface again by the same means which accomplished its descent. The sand is then allowed to consolidate round the pile, which it does rapidly and satisfactorily. No difficulty is experienced from boulders or large stones for, if met with, they can be displaced or lowered by a preliminary action of the jet below them.

This hydraulic method of sinking piles is often used in conjunction with the falling ram in earth of a compact nature. The pile in this case is naturally furnished with a pointed end, preferably conical.

Timber piles are universally in evidence, but iron and concrete piles also

have their uses. The drawback to timber piles is that, although extremely durable while completely protected from atmospheric influence, they are very susceptible to decay in air and, more particularly, "betwixt wind and water," and to perish from the attacks of insects.

Iron piles with pointed ends, and concrete piles on the Hennebique system (figs. 26 and 27), (vide also Chap. vii.) should only be driven through the interposition of a wooden dolly (fig. 28).

For untrustworthy strata of indefinite depth, piles, whether of wood or iron, are occasionally furnished with a broad screw end to the extent of a single turn or slightly more. This considerably increases the bearing surface. Such piles have to be lowered by rotation, either by means of manual, animal, or mechanical power

Ram and Fall.-Piles may theoretically be driven at the same rate with a light ram and a long fall as with a heavy ram and short fall, but the second method is preferable in practice. A long fall means greater oscillation in the ram and a consequent jar in the delivery of the blow, which tends to rupture the pile. From extensive experience in the driving of wooden piles, the author finds a monkey of 1 ton weight, with a fall of 8 or 10 feet, a very suitable combination. For concrete piles on the Hennebique system, even less fall is desirable, and a monkey of 2 tons, with 4 feet drop, has answered very satisfactorily at Southampton.

Quiescence. If the driving of a pile be interrupted for a short time, it is found that the resistance offered to driving is materially increased. Piles which have been left partially driven overnight have exhibited a resistance nearly three times as great on the resumption of work in the morning. This