Diving, aside from the pleasure afforded to good swimmers, is important in many different industries, particularly in fishing for pearls, corals, sponges, etc.

Without the aid of artificial appliances a skilful diver may remain under water for two, or even three minutes; accounts of longer periods are doubtful or absurd.

[Ill.u.s.tration: LONGITUDINAL SECTION OF HOPPER DREDGER, Employed on the River Clyde

The Vessel steams to place of working and is moored by the Steam Winches A A at bow and stern to buoys, the Bucket Ladder B is then lowered by steam power, and thereafter Buckets set in motion by gearing C C. The depth of water at which the Bucket Ladder dredges is regulated by the Hoisting Shears and Chain Barrel D D, driven by shafting E E from the Engines. The Buckets discharge the material by the shoot F into the Hopper G. The dredged material is discharged by the doors of the Hopper being opened by the Lifting Chains H H. These doors are hinged on to the side of Vessel, and suspended at centre by the Lifting Chains, which are connected to geared Crab Winches I I.]

[Ill.u.s.tration: SECTIONAL VIEW OF DIVING BELL AND BARGE, Employed on the River Clyde

All the appliances are worked by steam, rendering manual labour unnecessary. A is the Bell, which is raised and lowered by means of the Chain and Steam Winch B. _c c_ are Seats within the Bell; _d d_, Footboards. E, Air-pipe entering the Bell at _f_, the air being supplied by Air-pump G driven by the Engine H. J is a Steam Crane for raising or lowering material. K K, Steam Winches for working moorings and shifting position of the barge.]

Various methods have been proposed and engines contrived to render diving more safe and easy. The great object in all these is to furnish the diver with fresh air, without which he must either make but a short stay under water or perish.

Diving bells have been used very effectively. A diving bell is a contrivance for the purpose of enabling persons to descend, and to remain, below the surface of water for a length of time, to perform various operations, such as examining the foundations of bridges, blasting rocks, recovering treasure from sunken vessels, etc.

Diving bells have been made of various forms, more especially in that of a bell or hollow truncated cone, with the smaller end closed, and the larger one, which is placed lowermost, open.

The air contained within these vessels prevents them from being filled with water on submersion, so that the diver may descend in them and breathe freely for a long time provided he can be furnished with a new supply of fresh air when the contained air becomes vitiated by respiration. This is done by means of a flexible tube, through which air is forced into the bell.

A form, called the "nautilus," has been invented which enables the occupants, and not the attendants above, to raise or sink the bell, move it about at pleasure, or raise great weights with it and deposit them in any desired spot.

How are Harbors Dredged Out?

There are several forms of mechanical, power-operated dredges. One of the most common is the "clam-sh.e.l.l" dredge, consisting of a pair of large, heavy iron jaws, hinged at the back, in general form resembling a pair of huge clam sh.e.l.ls. This with its attachments is called the grapple. In operation it is lowered with open jaws, and by its own weight digs into the ground that is to be excavated. Traction is then made on the chains controlling the jaws, which close; the grapple is hoisted to the surface and its contents discharged into scows alongside the dredge.

The dipper dredge, an exclusively American type, has a bucket rigidly attached to a projecting timber arm. In operation the bucket is lowered and made to take a curving upward cut, thus dipping up the bottom material, which is discharged through the hinged bottom of the bucket.

The pump or suction dredge operates by means of a flexible pipe connected with a powerful centrifugal pump. The pipe is lowered into contact with the bottom to be excavated and the material is pumped into hopper barges or into a hopper-well in the dredge itself.

The center ladder bucket dredge operates by means of an endless chain of buckets moving over an inclined plane, which in structure is a strong iron ladder, one end of which is lowered to the sea bottom. The steel buckets scoop up the material at the bottom of the ladder, which they then ascend, and are discharged by becoming inverted at the upper end of the ladder. This dredge is the only one found satisfactory in excavating rock.

How is a Razor Blade Made?

The best scissors, penknives, razors and lancets are made of cast steel.

Table knives, plane irons and chisels of a very superior kind are made of shear steel, while common steel is wrought up into ordinary cutlery.

In making razors, the workman, being furnished with a bar of cast steel, forges his blade from it. After being brought into true shape by filing, the blade is exposed to a cherry-red heat and instantly quenched in cold water. The blade is then tempered by first brightening one side and then heating it over a fire free from flame and smoke, until the bright surface acquires a straw color (or it may be tempered differently). It is again quenched, and is then ready for being ground and polished.

The Story of the Tunnels Under the Hudson River[58]

The building of the Hudson River tunnels was probably one of the most daring engineering feats ever accomplished. As is well known, the Hudson River, for the length of Manhattan Island, is approximately a mile wide, reducing in width at the Palisades north of Hoboken. In consequence of the unusual geographical situation, all trunk lines and other transit facilities in New Jersey terminate on the westerly sh.o.r.e of the Hudson, and pa.s.sengers were of necessity compelled to use ferries to reach New York. A conservative estimate, which was confirmed by various counts, indicates that, prior to the construction of the tubes, the annual pa.s.senger traffic between New Jersey and New York was 125,000,000, and to handle this great volume of traffic the transportation companies a.s.sembled in the Hudson River a fleet of rapid ferry boats and maintained them up to the highest and most modern standards. But this very expeditious ferry service was not enough, and for many years there was a demand for facilities for more rapid transportation of the tremendous population residing in the suburban district of New Jersey tributary to New York City. As far back as 1873, a company had been organized to construct a tunnel under the river, but had met with numerous and most discouraging difficulties and obstacles, so that it was finally compelled to abandon the work, although it succeeded in building a considerable length of structure. Efforts were made at various times after that date to revive the work, with little or no results. In 1902 it was resumed, however, and a few years later was pushed to a successful end.

During the undertaking, more than 40,000 men were engaged in air-pressure work and there were many thousand more who did not work under air pressure. This vast army of men consisted of all nationalities and all grades and conditions of labor. The skilled tunnel workmen are men of character and ability, usually young, of good intelligence and sound of body, without a streak of fear or cowardice in their makeup.

All of those characteristics are essential to under-water air-pressure work.

As is quite generally known, air pressure and tunnel shields were used in all of the under-water work. It might be well to here correct the misconception which exists in the minds of many, that the use of air pressure for such purposes is something comparatively new. This is not the case. The use of air pressure was a very early invention, and it is a matter of record that in 1830, Admiral Cochrane, afterwards Lord Dundonald, was granted letters patent for the use of air pressure in tunnel construction. The modern engineer has merely developed the art to a high degree.

The method of construction used in the Hudson River tunnels has been designated the "shield method." In this type of construction, the primary part of the tunnel structure consists of an iron sh.e.l.l, formed of segmental rings, bolted together through inside f.l.a.n.g.es, and forming a large articulated pipe or tube, circular in section. This iron sh.e.l.l is put in place segmentally by means of a shield, an ingenious mechanism which both protects the work under construction and a.s.sists in the building of the iron sh.e.l.l.

[Ill.u.s.tration: THE NEW SHORT CUT TO NEW YORK

Hudson River Tubes of the Hudson & Manhattan R. R. Co.]

A tunneling shield consists essentially of a tube or cylinder slightly larger in diameter than the tunnel it is intended to build, which slides over the exterior of the finished lining like the tubes of a telescope.

The front end of this cylindrical shield is provided with a diaphragm or bulkhead in which are apertures which may be opened or closed at will.

Behind this diaphragm are placed a number of hydraulic jacks, so arranged that by thrusting against the last erected iron ring the entire shield is pushed forward. The hind end of the shield is simply a continuation of the cylinder which forms the front end, and this hind end, or tail, always overlaps the last few feet of the built-up iron-sh.e.l.l tunnel.

When the openings in the bulkhead are closed, the tunnel is protected from the inrush of water or soft ground, and the openings may be so regulated that control is maintained over the material pa.s.sed through.

After a ring of iron lining has been erected within the tail of the shield, excavation is carried out ahead. When sufficient excavation has been taken out, the jacks are again extended, thus pushing the shield ahead, and another ring of iron is erected as before.

[Ill.u.s.tration: ONE OF THE SIXTY-SEVEN-TON TUNNEL SHIELDS]

For the erection of these heavy plates, a hydraulic swinging arm, called the "Erector," is mounted, either on the shield itself or on an independent erector platform, according to conditions. This erector approaches closely the faculties of the human arm. It is hydraulically operated and can be moved in any desired direction. This method of construction can be followed in almost every kind of ground that can be met with, and it is especially valuable in dealing with soft, wet grounds. In pa.s.sing through materials saturated with water, the shield is a.s.sisted by using compressed air in the working chamber.

[Ill.u.s.tration: CUTTING SHIELD HEAD]

The employment of compressed air under such conditions is really a rather simple thing in itself, and means merely that the pressure of air in the chamber where men are working is maintained at a point sufficient to offset the pressure of the hydrostatic head of water and thereby prevent its inflow. A crude comparison may be made by saying that if the ceiling of a room was weak and threatening to fall--if we filled the room with sufficient pressure of air, it would support the ceiling and prevent it falling in. In tunnel work, air is supplied under compression from the mechanical construction plant located on the surface, and the pressure of air maintained in the working chamber is determined by the depth of the work below tide level, as the hydrostatic head increases with the depth.

Control of air pressure is never entrusted to any but the most reliable, competent and experienced man, as it is of the utmost importance that air pressure be maintained properly. The first impulse of an inexperienced man, should he notice an inrush of water, would be to increase the air pressure, which might be a very dangerous thing to do.

An experienced man, however, would very likely first lower his pressure in such an emergency, and then put up with the nuisance and difficulty of having a good deal of water in his working chamber. By doing this, he would permit the greater external pressure to squeeze the soil into the leaking pockets and thereby choke the leak.

[Ill.u.s.tration: Ap.r.o.n IN FRONT OF SHIELD, FIVE MINUTES BEFORE SHOVING]

To improperly or inopportunely raise the air pressure would be quite likely to result in the air blowing a hole through the roof of the tunnel heading, allowing all air pressure to escape, and permitting an uncontrollable volume of water to rush in and flood the work.

The outer sh.e.l.l of the tunnel shield is composed of two- or three-ply boiler plates, and the interior is braced with a system of steel girders. The shields used weighed approximately sixty-seven tons each.

Sixteen or eighteen were used. To move the shield forward, each shield was equipped with sixteen hydraulic jacks, arranged around the shield circ.u.mferentially. These jacks were controlled by a series of valves, which were so designed that any one jack or any set of jacks desired could be operated. This was necessary as the direction of the shield was, as it were, guided by the pressure of the jacks. When it was desired to alter the direction of the shield, either upwards or downwards, or to the right or left, the jacks on the opposite side to which the shield was to point, were operated. The hydraulic pressure operating these jacks was 5,000 pounds per square inch, and the total energy, when all jacks were employed at the same time, was equivalent to 2,500 tons, which was equal to eleven tons per square foot of heading.

[Ill.u.s.tration: CUTTING EDGE OF SHIELD IN NORTH TUNNEL]

Air pressure used to prevent the inflow of water and soft dirt varied from nothing up to forty-two pounds, although a fair average throughout was thirty-two pounds. It varied, of course, according to the condition encountered.

The working chamber is the s.p.a.ce between the tunnel heading where work is in progress and the air-lock. The air-lock is a device used for the purpose of enabling workmen and materials to pa.s.s from the portion of the tunnel where the atmospheric pressure is normal into the portion where the air pressure is greater than normal; that is, the working chamber. The air-lock is a cylinder, usually about six feet in diameter and twenty feet in length, with a heavily constructed iron door at each end. This lock is placed horizontally in the tunnel at such a level as the conditions of the work necessitate, but usually near the bottom, and around this cylinder, and completely filling the cross-section of the tunnel, a concrete bulkhead is built and is known as the lock bulkhead.

The two doors open in the same direction; the one at the normal pressure end opening into the cylinder, and the one at the heading end opening away from the cylinder. One door is always closed, and both doors are closed during the operation of entering or leaving the air-pressure section.

Going into the air pressure, the door at the heading end is held closed by the pressure of air against it while one is entering the lock, after which the outer door is also closed. A valve is then opened which permits the air to flow from the working chamber into the lock, until the lock becomes filled with air of the same pressure as exists in the heading. As soon as the pressure is thus equalized, the door at the heading end can be opened and the workmen pa.s.s into the heading. Going out, the operations are simply reversed. After the heading door is closed, with the workmen in the air-lock, a valve is opened which permits the air in the lock to exhaust into the normal air, until the pressure within the lock reduces to the same as that outside, when the outer door can be opened and persons inside the lock pa.s.s out. Both operations must be gradual, as a sudden change from normal to high pressure, or _vice versa_, would be very dangerous to anyone.

[Ill.u.s.tration: SHIELD CUTTING EDGE BREAKING THROUGH WALL AT SIXTH AVENUE AND TWELFTH STREET, LOOKING SOUTH, OCTOBER 23, 1907]

In tunneling under the river, nearly every conceivable combination of rocks and soils were met, but for the most part the material was silt.

In such material, with a pressure of 5,000 pounds per square inch on the shield jacks, the shield was pushed through the ground as though one pushed a stick into a heap of snow, pushing aside the silt, and thus obviating the necessity of removing any excavated material. Sand or gravel, or any material which would not flow or become displaced by the shield, of course, had to be excavated ahead of the shield, and removed from the heading prior to pushing it forward. In the silt the most satisfactory and economic progress was attained, and a record was made of seventy-two feet of finished tunnel, completely lined with iron, in one day of twenty-four hours.

The most difficult combination that had to be dealt with under the river was when the bottom consisted of rock and the top of silt and wet sand.