But on the following morning, when Colin accompanied Colonel Ossington in a walk round the garden, a new light seemed to come to him.  
They were passing the little postern of which so much had been said—the postern through which, as the boy declared, he had himself seen the apparition of Neil Leslie disappear on the previous night. Here Colin now stood. He stamped his feet upon the ground.
 
"Listen!" he said. "Do you hear anything?" He stamped once again. "I've often thought, as I have passed this spot, that the ground seems to give back a hollow sound."
 
"And if it does, what of it?" asked Colonel Ossington.
 
"Well," said Colin, with a curious lift of his eyebrows, "I was thinking that it is just possible there may be some cave, or passage, or cellar under here; and that perhaps it was down there that the guns and things you were telling us of last night were stored."
 
"You may be right," smiled the colonel, "but I don't see that it matters very much now. It's so long ago, you know."
 
"Yes," went on Colin, "but I should like to find out, all the same. I have often thought of it before—of the underground passage, I mean. Most castles in Scotland have underground passages somewhere, and Castle Leslie can scarcely be an exception. At one time I thought I had found a way into this one." He pointed up to the top of the ivy-covered wall. "You see the place where that buttress ends?" he asked. The colonel nodded. "Well, last spring a jenny wren built her nest up there. I wanted to get it. I climbed up from the inside of the ruin, and crept along the top of the wall. I had got as far as where the nest was when, leaning over to reach it, I felt one of the big stones give way beneath me. I held on by the ivy; but the loosened stone fell with a crash to the ground. I didn't look where it fell. I was only thinking of how I should get down with the nest. But a day or two afterwards I was coming through the place that used to be the guard-room in the old days, before Hawley's dragoons burnt this part of the castle down, and I saw the stone lying there. It wasn't smashed; but it had smashed the flagstone that it had fallen upon. Some parts of the flagstone had dropped through, right down into a sort of black well. I did not try to open the well; although I should have done if any other boys had been here to help me. But this morning I thought of it again in connection with your story—"
 
"I understand," interrupted the colonel. "You think it may have been down there that old Sir John Leslie hid the arms for the rebels, eh? Well, let me see this fancied entrance to the subterranean passage. Where is it?"
 
"It's through here," said Colin. And he led his companion through the postern gate into a large roofless room.
 
In one of the corners there was a heap of garden refuse, covered by a thin layer of melting snow. Colin took an old spade and industriously cleared the rubbish away. Presently he revealed a large cracked flagstone. He went down on his knees and busily endeavoured to dislodge one of the broken fragments. He scraped and tore and pulled at it to no purpose. Then he stood up and stamped upon it. The rattling of loose earth underneath encouraged him to continue.
 
"Can you find such a thing as a pickaxe?" questioned Colonel Ossington.
 
Colin shook his head, but ran, nevertheless, in search of some such instrument, returning some minutes afterwards with a heavy sledge-hammer. With this he opened an assault upon the flagstone, and soon succeeded in loosening one small fragment. A small brown rat darted out from the excavation and scampered across the uneven floor.
 
"Wait!" cried the colonel; "lend me the hammer. Let us try first to remove this smaller stone, then we can better get at the larger one."
 
He took the sledge-hammer, raised it over his shoulder, and brought it down with a well-directed blow upon the smaller stone, splitting it. A second blow broke it into splinters. These he removed. Beneath them he discovered the end of a rusty bar of iron that was shot like a bolt through an iron ring. The bar seemed to extend under the larger flagstone, supporting it through its centre of gravity. For many minutes he hammered at the rusty iron, and with each blow the flagstone trembled on its axle and a shower of loosened stones and gravel fell into the depths below. With each development the old soldier's energy increased, while Colin looked on absorbed in boyish expectation.
 
At last the corroded bar broke. The flagstone collapsed and slipped a few inches into the void, where it was arrested by some obstacle. Its removal revealed an irregular opening, some two feet in diameter.
 
 
"You were right, boy," remarked the colonel; "there is indeed a secret chamber here, and this is, or once was, its entrance. See! the flagstone has formed a sort of trap-door. It may have been opened by a spring set under the smaller stone at the side. Look down there; you can see the edge of one of the stone stairs."
 
"Can we get down?" asked Colin.
 
"It is possible, I think," returned the old soldier. "But we should require a lighted lantern. Could you fetch one?"
 
Colin ran off. He was absent some ten minutes. During that interval Colonel Ossington contrived so to force back the broken flagstone that it left an opening sufficiently wide to admit his body. He went upon his knees and thrust his feet into the cavity, descending step by step until his eyes were on a level with the paved floor. There he waited, resting with his hands on the second step. The fingers of his right hand touched something that was softer than the cold stone. He gripped it and drew it forth into the fuller light. It was a fragment of mouldy cloth or felt. Attached to it was a disc of tarnished metal upon which the figure "4" was embossed.
 
"God!" he exclaimed, "it's the badge of the Fourth Foot."
 
He tore off the badge and thrust it into his pocket. At this moment Colin Leslie appeared with the lighted lantern, and accompanied by his grandfather.
 
"I am glad you have come too, Sir Donald," said the colonel somewhat absently.
 
"What boy's adventure are you contriving now, colonel?" demanded Sir Donald. "One would think that you had gone back to your childhood."
 
"Not quite so far back as that," returned the old soldier grimly, "but my mind has indeed gone back to my young manhood. Give me the light, Colin," he added, turning to the lad. "I had better, perhaps, go down in advance."
 
 
"Colin handed him the lantern."
Colin handed him the lantern and stood at the top of the steps watching him slowly and cautiously descend. The light flickered upon the damp moss-grown stones of the walls that formed the sides of the narrow stairway. It went down and down, growing gradually dimmer and dimmer, until at last it died away. The old grandfather and Colin waited, listening. They faintly heard the tread of the colonel's spurred boots echoing hollowly in the darkness. Once they heard him cough, and then all was silent. The minutes slowly passed. Sir Donald grew a trifle nervous, his nervousness being indicated by the impatient tapping of his foot.
 
"Listen!" cried Colin. "I heard something fall—something that rattled." He knelt down and peered into the opening. "I hear him walking," he whispered. "He's coming nearer now. Now he has stopped. Now he is coming on again. He's on the stairs. He's carrying something that knocks against each step. I can see the reflection of the light now. And now here's the lantern." The boy drew back. "Mind your head, colonel, or you'll knock it," he cried.
 
Colonel Ossington did not require the caution. Bending his head, he crept upward, holding the lantern in his extended hand. Presently his face appeared in the aperture. It was ghastly white, and his eyes stared wildly. He drew a deep breath of the fresher air.
 
"You had better come down," he said, glancing up at Sir Donald Leslie; and drawing his left hand upward, he cast an old and rusty broadsword at the old man's feet. Sir Donald glanced at the weapon and kicked it aside.
 
"Come!" reiterated the colonel in a voice of authority, and the grandfather slowly obeyed. Colin followed him down the steps, although he was aware that he had not been included in the command. Perhaps he would have been wiser to remain where he was, but his boyish curiosity and love of adventure overcame his caution. Step by step they descended into the gloo............