the door ahead of you swings open and you see the long, dim vista of the tunnel ahead. You enter into what appears to be the hollow carcass of a monstrous serpent, whose ribs stretch before you, round after round, apparently without end. Yet not very far away the bed of the river is flowing like a stream of black lava through a door in the shield, and the "sand-hogs," as tunnel workers are called, are loading it into little cars that stand on a track laid on the floor of the tunnel.

And now your guide explains to you the mystery of the compressed air. It is the air, driven at a pressure of thirty pounds to the square inch against it, that holds the fluid bed of the river in check. Often the men work in a pocket in front of the shield, and while they work the compressed air holds up the face of the excavation, blows out the water, and keeps the silt dry. But here is the danger. The pressure of the river varies with its depth-that is, with the length of the column of water that rises from the excavation to the surface of the stream; the pressure of the air is uniform at all points of the face of the excavation, and can therefore balance the pressure of the river only at a single point. Where the tunnel is twenty-three feet in diameter, the difference in the pressure

of the river at the upper part of the shield and at the lower part may be as much as nine to eleven pounds. Now, ordinarily, the pressure of the air is made to balance the maximum pressure of the river at the bottom of the shield. So long, therefore, as the face of the excavation is composed of fairly compact material, everything goes well. But let the shield be driven into a stratum of over-liquid silt or quicksand, and the face of the excavation at the top of the shield is pretty sure to give way. Then the air "blows out" with a burst, its pressure within the shield is rapidly reduced, the equilibrium between it and the river is overcome, and the water rushes in like a hungry monster. Against this emergency a safety curtain is erected within the tunnel tube, but frequently enough the men cannot outdistance the river in their rush to cover, and thus many a good life has been lost.

But your guide will assure you that the men like their work. The engineer who conducted me through the tunnel told me how, when the two Pennsylvania tubes were approaching the line that separates New Jersey from New York, there was great rivalry between the forces in each of them to cross the border first. Day and night the men in either tube could hear the shouts and

the singing of their neighbors beneath the river, and the cry of triumph that went up when the men in the southern tube won the race.

As you make your way out of the tunnel back into the open air, you feel that you have been in the presence not so much of a work of giants as of heroes. Every day the men who enter the tunnels take their lives in their hands, and the delight which they experience in their work is due to their consciousness of participation in an enterprise the successful conclusion of which will bring a boon to humanity.

I have dwelt upon the Pennsylvania tunnel and the New York Central subsurface yards because they are typical of the major improvements now under way. If the reader will turn to the accompanying map of New York, he will see the larger scope of the works that are changing the metropolis from one of the most awkward and difficult of access to perhaps the most ample and convenient terminal in the world. Under the Hudson, westward from Thirty-second Street run the two great twenty-three-foot tunnels of the Pennsylvania, already in part described. Further south, at Christopher and again at Cortlandt Street, are the four so-called McAdoo tunnels, which, it is announced, are to be used by the Erie, the Lackawanna, the Central Railroad of New Jersey, and by the suburban trains of the Interborough-Metropolitan Rapid Transit Company. From Battery Park, in Manhattan, to Joralemon Street, in Brooklyn, stretches the splendid tunnel built for the use of the Metropolitan Company by the city. Eastward from Thirty-fourth Street four Pennsylvania tunnels plunge beneath the East River to East Avenue in Long Island City. And above all these, in the reefs of Blackwell's Island, east of Forty-second Street, lie the two Belmont tunnel tubes. These tunnels, with their magnificent terminals at Morton and Cortlandt Streets, together with the great New York Centrai and Pennsylvania sub-surface

yards, constitute the most important contributions which have thus far been made to the solution of New York's transportation problem.

But other improvements are in process of execution by the railways that will be hardly less important in their ultimate effect upon the commerce of the city, and to these I shall give a concluding word. Across the Hudson, at Greenville, on the long peninsula that stretches down between New York and Newark Bays, the Pennsylvania has for several years been building a freight yard which, when it is completed, will accommodate forty thousand cars. This yard is supplemented by a lighterage pier two thousand feet long, extending out into the water of New York Bay, and with three float bridges, equipped with the most modern and ingenious electrical machinery for the transfer of freight trains to the barges that carry them across to Bay Ridge in Long Island. At Bay Ridge another freight yard with lighterage piers and bridges has been laid out; and at Sunnyside, between Jackson and Morton Avenues in Long Island City, the Pennsylvania has just begun the excavation of what will be possibly the largest yard devoted to the exclusive use of the passenger service in the country. This Sunnyside yard, like the New York Central and Pennsylvania yards in Manhattan, will lie below the level of the city streets, which will be carried across it on steel viaducts or under it in tunnels, and its business will be transacted entirely by electricity. When completed, this yard will contain forty-five miles of track, capable of accommodating a few more than two thousand coaches. Finally, to the north of these yards a bridge has been planned that will carry traffic across Ward's and Randall's Islands to Port Morris in the Bronx. These Long Island terminals, if they will not be among the most conspicuous, are destined to be among the most useful, of the new gates of New York.