No two cities are exactly alike. New York, Boston, London, Paris; all these have their subways, giant tunnels through which thousands upon thousands of workers are hurled to their day of toil. The metropolis of our story has no subway; yet far down beneath its busy streets forty, fifty, sixty feet underground, one still finds life. Forty miles of tunnel, a great spider-web network, pass beneath this city.
 
It was into one of these tunnels that Curlie Carson, while in pursuit of the man who had snatched the precious package from beneath his arm, had entered.
 
The roaring sound he had heard was the approach and the passing of a tunnel train. But such a train as it was! A narrow, box-like electrical engine shooting out purple sparks; a man with his hand on a lever; a dozen cars the size of a city dumpcart, only narrower and deeper—this was the train; for these trains carry only freight.
 
Day and night, year in and year out, this endless procession of tiny trains carries coal to the heating plants of giant skyscrapers and bears the cinders away. By this route, too, thousands of tons of merchandise, shoes and suits, cheese, cabbages, silverware, and socks find their way to the great city department stores or from factory to ship or train.
 
How many dwellers in the great city are conscious of this life that throbs on and on beneath them as they walk the city’s streets? Perhaps one in a hundred. Curlie Carson had not so much as heard of these tunnels. Yet he was passing down the narrow stairway that led to a small landing platform. In less than a moment he would be called upon to make an instant decision which might spell victory or utter defeat. And this decision would have to be made in ignorance of that which lay beyond him in those dim caverns.
 
It is often so in life. Always we are preparing ourselves for an emergency. Are our eyes bright, our minds clear and free from low thoughts that drug the soul? If so, then we are ready for the sudden, the unexpected. Curlie was ready.
 
The thing that happened was this: As the boy came stumbling down the last twenty steps of that long stairway, he heard again the ever-increasing rumble that told of an approaching train. As he stepped at last upon the dimly lighted platform he saw the man he sought at the far end. The package was under his arm. He was looking the other way.
 
“Now I have him!” he thought, and his heart beat a loud tattoo against his ribs.
 
Curlie was slim but strong, and agile as a cat. He had a clear mind. He kept himself fit. He had no dread of an encounter.
 
At this time at least there was to be no encounter. For as Curlie sprang forward the man turned and saw him. At the same instant an empty train came rattling around the curve.
 
Urged on by who knows what desperate need, the stranger played a bold hand. The cars were moving rapidly. Not three feet above them, charged with enough electricity to kill a hundred men, ran a high tension wire. Despite all this, the stranger made a sudden leap, caught the side of a steel car, struggled desperately, regained his balance, and disappeared within its depths.
 
At that instant Curlie felt hope vanish. Despair gripped his heart. The man with that package, the loss of which meant to him dismissal, disgrace, perhaps a prison, was rattling away. Who could tell whither the car was bound? Knowing nothing of these tunnels, the boy could not so much as guess.
 
All this passed through his mind like a flash. Cars still bumped and rattled by. It was a long, empty train.
 
The boy looked up the tunnel. Except for a narrow space above, the cars filled the tunnel completely. If he attempted to duplicate that man’s feat and failed in the least detail, he would meet instant death.
 
Yet he could not surrender. Too much was at stake. He had risked too much for others. Now he must risk more for his own honor. With a brief prayer for guidance and protection, he put out both hands, gripped the edge of a steel car, swung his feet high, caught the gleam of copper wire above him, felt the shudder of steel beneath him and then fell with a force that stunned him to the bottom of the car.
 
For a long time after that the train, bearing its strange cargo, rattled on and on into the perpetual night that is the tunnel system beneath the great city.
 
The various sections of this intricate system are placed in exactly the same manner as are the streets of the city. Indeed, at their intersections they are plainly marked: “State and Madison,” “Wabash and Monroe,” “Michigan and Jackson.” Had Curlie dared to raise himself from the floor of the car he might at once have determined his position in relation to the city above him.
 
The deadly copper wire warned him down. “Besides,” he reasoned, “what’s the good? One does not rise through fifty feet of clay and sand, rock and cement to burst up from the street like a chick from an egg.”
 
There is in this tunnel an intricate system of signal lights. Red just ahead warns the operator of the engine th............