In such cases, and there were many of them, the upper section of soft ground was first excavated and the exposed face securely supported with timbers ahead of the shield, and the rock underlying then drilled and blasted. This was very tedious and expensive work. Exceedingly small charges of dynamite had to be used and the procedure conducted with the utmost caution.

In the course of their progress, the shields were subjected to the most intense strains and hard usage, as may well be imagined. One of the shields is ill.u.s.trated. It was used to construct the south tunnel of the up-town pair of tubes, and pa.s.sed from under the Hudson River, through Morton, Greenwich and Christopher Streets, into Sixth Avenue, and north to Twelfth Street, a total distance of 4,525 feet, of which 2,075 feet was through rock overlaid with wet sand. During the progress of this shield, 26,000 sticks of dynamite were exploded in front of the cutting edge, causing great damage to the structure of the shield, so that when it arrived at its destination at Sixth Avenue and Twelfth Street, it was in such a condition of distortion that it was with difficulty that the tunnel lining could be erected behind it.

[Ill.u.s.tration: NORTH TUNNEL, SHOWING COMMENCEMENT OF NEW WORK]

In pushing a shield forward with the battery of powerful hydraulic jacks, each advance is of two feet, and must be followed immediately by installation of the permanent lining in the rear. In the early days, brick work was used for lining, and in recent years it has also been used to some extent, but even with the use of quick-setting Portland cement, neither brick work nor concrete has proved successful for subaqueous work, as the cement cannot reach the required strength within the time it is feasible to leave the shield standing before advancing it again.

[Ill.u.s.tration: HOLE BROKEN THROUGH THE SOUTH TUBE OF THE NEW YORK AND JERSEY TUNNEL LOOKING WEST]

During the early work on the north tube of the uptown tunnels, a point was reached where the rock was sixteen feet above the bottom of the tunnel, and the overlying silt was in a semi-fluid state. Five barges of clay had been dumped in the river over this point to make a roof for the tunnel, but the fluid clay could not be controlled, and crept through the doors of the shield. After trying all known methods to get through, it was decided to bake this wet clay by means of intense heat. Two large barges of kerosene were sent into the tunnel, and an air pipe connected to them. Fine blow-pipes were also attached, and the fire from the blow-pipes was impinged on the exposed clay until it became caked sufficiently dry and hard to overcome slipping. It required eight hours of this baking to dry the clay hard, and, during this period, water had to be played continuously on the shield to avoid damage due to the high temperature. It is believed that this was the first time that soft material met with in tunneling under a river has been solidified by means of fire. Seven days after pa.s.sing this troublesome point, the rock suddenly disappeared and the work proceeded without further trouble.

[Ill.u.s.tration: NEW YORK AND NEW JERSEY TUNNEL SHOWING SIGNAL AND CAR]

Another unusual situation occurred in the south tunnel of the uptown tubes. When the shield had advanced 115 feet from the Jersey side, the night superintendent in charge of the tunnel work, in his anxiety to push the work, disobeyed instructions, and the tunnel got away from him and was flooded, and his men had a narrow escape with their lives. In order to regain the tunnel, several schemes were considered, including that of sending a dredge through to dredge out the bed of the river just in advance of the shield, a sufficient depth to enable a diver to go down and timber up the exterior opening of the doorway, where the silt and mud had come through and filled the tunnel. This plan had to be abandoned, as the river above was almost entirely occupied by shipping that could not be interrupted.

[Ill.u.s.tration: AN X-RAY VIEW OF A BUSY HALF-MILE UNDER THE GROUND ON THE JERSEY SIDE OF THE HUDSON RIVER]

[Ill.u.s.tration: CROSS-SECTION ON SIXTH AVENUE AT THIRTY-THIRD STREET, NEW YORK

1. FOOT Pa.s.sAGE 2. MANHATTAN ELEVATED RAILROAD 3. STREET SURFACE AND METROPOLITAN STREET RAILWAY 4. NEW RAPID TRANSIT SUBWAY 5. HUDSON AND MANHATTAN RAILROAD STATION 6. PENNSYLVANIA RAILROAD TUNNEL]

Finally the difficult situation was met by obtaining two large and heavy mainsails, which made a double canvas cover measuring about sixty by forty feet. This canvas cover was then spread on a flat barge, small sections of pig iron being attached around the edges of it. Ropes were carried to fixed points to hold it in exact position. The barge was then withdrawn, and the canvas cover dropped to the bed of the river, and, most fortunately, it settled over the point where the leak had occurred, and a large number of bags of dirt were then deposited on it. An opening was then made in the bulkhead of the tunnel below, and for eight days material, under hydrostatic pressure, forced its way into the tunnel, where it was loaded on cars, and finally the canvas was drawn into the hole, stopping it up. Additional material was then deposited into the river to fill the cavity, and finally the tunnel was recovered, pumped out and work resumed. This event is of somewhat historical interest, in that the two mainsails which were used were procured from the owner of the famous American cup defender, the well-remembered "Reliance."

Probably the most unique and interesting pieces of construction are the three junctions on the Jersey side of the river, where the uptown tunnels from New York diverge, north to Hoboken and south to Jersey City and New York downtown. For safe and expeditious operation of trains, where the schedule is only one and one-half minutes, it was imperative that grade crossings should be avoided. By grade crossings is meant the tracks of one service crossing the tracks of another service at the same grade. At the point in question, this was a knotty problem to solve, owing to the unusual operating conditions which had to be met, there being six separate and distinct operating cla.s.ses of trains to be handled around this triangle.

To meet this situation, three ma.s.sive reinforced concrete caissons were built on the surface. They are practically large two-story houses, each being over one hundred feet in length, about fifty feet in height, and about forty-five feet in width at their widest point. The bottom edges were sharp, and, with the use of air pressure and great weights, the three structures were sunk in the ground to the same grade as the intercepting tunnels, and the tunnels were then driven into them.

Particular attention should be given to the Jersey City to Hoboken tube, in the lower part of the caisson in the foreground, in the accompanying ill.u.s.tration, which curls around the Hoboken to Jersey City tube, and rises to the elevation of, and connects into, the New York to Hoboken tube, at the caisson in the background, at the left of the ill.u.s.tration.

Very few of the people who travel through the tube are probably aware of such manipulation. At the same time, the arrangement absolutely avoids any grade crossing whatever, and without such an arrangement of tracks the road could not be operated with trains run so closely together as under the prevailing system.

In constructing the river tunnels the work was carried on simultaneously from opposite sides of the river, the tunnels meeting under the river, and it is interesting, if not remarkable, when one considers the difficulties under which the engineering work had to be carried on, to note that the tunnels met with practically absolute accuracy.

What Causes Floating Islands?

A floating island consists generally of a ma.s.s of earth held together by interlacing roots.

They occur on the Mississippi and other rivers, being portions of the banks detached by the force of the current and carried down the stream, often bearing trees. Sometimes such islands are large enough to serve as pasture grounds.

Artificial floating islands have been formed by placing lake mud on rafts of wicker-work covered with reeds. They were formerly used in the waters around Mexico, and may be seen in Persia, India, and on the borders of Tibet. On these the natives raise melons, cuc.u.mbers and other vegetables which need much water.

Pictorial Story of the Airship

[Ill.u.s.tration: A "PUSHER" OF SEVERAL YEARS AGO, WITH MANY OF THE MORE PROMINENT AIR-MEN

_Courtesy of The Curtis Aeroplane Co._]

[Ill.u.s.tration: _Courtesy of The Curtis Aeroplane Co._

UP-TO-DATE TWIN MOTORED MILITARY TYPE TRACTOR--200 H. P.]

[Ill.u.s.tration: _Copyright by Underwood & Underwood, N. Y._

THE FIRST PLANE TO CROSS THE ATLANTIC

The honor of being first to make the journey from America to Europe by airship fell to Lieut.-Commander A. C. Read, who piloted the U. S.

seaplane, NC-4, from Newfoundland to Lisbon, Portugal, with a stop at the Azores. The photo shows Lieut.-Commander Read and the seaplane, NC-4, in readiness for their long trip, which began May 16, 1919, and ended May 27th.]

[Ill.u.s.tration: _Copyright by Underwood & Underwood, N. Y._

THE FIRST FLIER TO CROSS THE OCEAN WITHOUT STOP

In this Vickers-Vimy aeroplane, Captain Alc.o.c.k and Lieutenant Brown made the first non-stop flight across the Atlantic on June 15, 1919, traversing 1,650 nautical miles in 16 hours 12 minutes.]

[Ill.u.s.tration: CHART OF THE TRANSATLANTIC FLIERS

This shows graphically the course of the transatlantic aviators. The U. S. navy seaplane was first to make the flight, leaving Newfoundland May 16, 1919, flying to the Azores in 15 hours, to Lisbon in 13-1/2 hours, and to Plymouth in 13 hours. Hawker, in a Sopwith aeroplane, left Newfoundland May 18th, and covered half the distance to Europe, but was compelled to descend. He was picked up by a steamer. Captain Alc.o.c.k and Lieutenant Brown made the first non-stop flight June 15th; and the British dirigible, R-34, made the first round-trip, leaving Scotland on July 2d.]

[Ill.u.s.tration: THE WRIGHT BROTHERS AND THEIR FAMOUS AEROPLANE

The machine is shown in action and resting on the ground. The pictures were taken during the army test flights at Fort Myer, Virginia.]

[Ill.u.s.tration: _Copyright by Western Newspaper Union._

FROM BRITAIN TO AMERICA AND BACK BY BALLOON

The great British dirigible, R-34, was the first lighter-than-air vessel to cross the Atlantic. She left East Fortune, Scotland, July 2, 1919, under command of Major Scott, and covered 3,200 miles to Mineola, Long Island, in 4 days 12 hours 12 minutes. The return journey was made in 3 days 3 hours 3 minutes. Note the piles of hydrogen gas bottles needed to replenish the gas supply.]

[Ill.u.s.tration: HIDE AND SEEK IN THE BALTIC

A Zeppelin flying over a British submarine in the stormy sea.]

[Ill.u.s.tration: A BATTLE OF FOUR ELEMENTS

British monitors sh.e.l.ling the German land batteries near Nieuport.

German submarines were actively engaged in trying to torpedo these monitors and the British monoplane was useful for giving the range to the ship and reporting the accuracy of the shots.]

[Ill.u.s.tration: ZEPPELIN DEVICE FOR DROPPING BOMBS

An armored car is suspended by three cables from the Zeppelin airship to a distance of several thousand feet below the monster aircraft, which is concealed in the clouds above. (_Sphere copr._)]

[Ill.u.s.tration: A BELGIAN MILITARY OBSERVATION BALLOON