"Where can he be going?" questioned Gysbert, as he poked up his head at the turn, and saw Dirk advancing straight on, apparently right to the wall itself. At that moment the man half turned his head and Gysbert ducked under hastily. When he again raised himself, to his amazement Dirk had disappeared as completely as though the earth had opened up and swallowed him.

"Has the rascal spread his cloak and flown over the wall, or has he changed his bodily substance and passed right through it, like the prince in the fairy tale?" demanded Gysbert of the air about him. But as it was plain this would bring no solution of the enigma, he cautiously crept toward the wall, determined by some means to solve the mystery.

From the turn of the canal to the wall was a distance of perhaps five hundred yards, an unoccupied space of ground like a meadow, broken by nothing save a little brook that connected with the canal. At the base of the wall this brook spread out for a space, like a miniature lake. Gysbert examined every inch of the ground attentively, without finding anything that might serve to enlighten him. At the face of the wall he stopped. Plainly no human being could scale at this point the high, smooth surface that confronted him. Dropping on his knees he examined the base. "Nothing here!" he muttered, and waded into the tiny lake that spread out before him.

Step by step he advanced, feeling carefully of the brick wall at every interval, to detect any possible weak spot, when suddenly his feet slipped into a deep hole, he was drawn under, and swept by the force of some swift current, through a small hidden aperture in the wall. When he came to the surface, he grasped at a projecting ledge, and tried to ascertain what had happened. It did not take him long to guess. The marshy land in and about Leyden was constantly intersected by the formation of new brooks and streams. Not infrequently they would undermine the very wall itself, and in times of peace, these defects were always carefully watched and remedied. But in the terrible strain under which the city had existed for the past months, this one had evidently passed unnoticed, and in truth, no one would have suspected its presence from the inside of the city, so well was it hidden by the little spreading lake.

"Now what ought I do next?" thought Gysbert when he had unravelled this mystery. "Without doubt this is Dirk's secret doorway, and how he discovered it the Evil One only knows! The question is, should I try to explore it before he is well out of the way? I would hardly care to meet him in this black hole! On the other hand, I don't believe he will remain in here a moment longer than he has to, and I'm freezing hanging here. I'll risk it!"

So saying he plunged into the grim cave, and commenced his journey through the base of the great wall of Leyden. To his disgust he found that the stream did not penetrate straight from side to side, but turned and pierced through the length of the wall for many yards. The way was difficult enough, since he had to fight every inch of his progress against the swift current, and once the water deepened to such an extent that he was forced to swim. Moreover, unwarmed by any sun it was icy cold, and his limbs grew numb and his teeth chattered.

For a moment panic seized him, and he felt sure he would never get out alive, but would drown in this horrible place. Then his natural courage again asserted itself, and he pressed steadily forward. At length the course of the hidden stream changed again, a faint glimmer of daylight appeared, and in another moment he stood outside the walls of Leyden, protected from the gaze of the Spanish camp only by a few bushes. No Dirk Willumhoog was to be seen, but there remained not a shadow of doubt that this was his mode of ingress to and exit from the city of Leyden.

Gysbert lay down in the sunlight, and warmed his numbed body in its welcome heat. In half an hour's time he had started on his return trip, and found it twice as easy as travelling in the opposite direction. Far from fighting the current he was helped along by it, and in a short time stood safe within the town again. Arrived there, another swim awaited him, for as he could not run through the town clad in nothing at all, he was obliged to take to the canal till he reached the spot where he had left his clothes. Once only he stopped to climb out and investigate the place where Dirk had sat examining his papers. As good luck would have it, he discovered hidden away in the grass where it had evidently fallen unnoticed, one of the scraps. On it were written a few words, evidently only a part of the whole, whatever that might have been. Gysbert read them and his eyes grew big with wonder, and then snapped angrily. "Ah, this is shameful!" he cried. "We'll see about this, Dirk Willumhoog, thou traitor as well as coward!"

With the paper in his mouth for safety, he plunged into the canal, swam to the point where he had left his clothes, flung them on hastily, and hurried home as fast as he could run.

"I shall have something to tell Jacqueline about this day's work!" he remarked to himself with great satisfaction.