"If you'll be warned by me, sir, you'll just stay quietly here."

"I must go down and see the matter out," was my answer. "Now listen to me, Henderson. I see that you are alarmed, and I don't wonder. Just stay quietly where you are and watch, but if I call come at once. Don't delay a single instant. Remember I am putting my life into your hands. If I call 'Come,' just come to me as quick as you can, for I may want help.

Give me that lantern."

He unhitched it from the wall, and taking it from him, I walked cautiously down the steps on to the line. I still felt curiously, unaccountably drowsy and heavy. I wondered at this, for the moment was such a critical one as to make almost any man wide awake. Holding the lamp high above my head, I walked rapidly along the line. I hardly knew what I expected to find. Cautiously along the metals I made my way, peering right and left until I was close to the fatal spot where the bodies had been found. An uncontrollable shudder pa.s.sed over me. The next moment, to my horror, without the slightest warning, the light I was carrying went out, leaving me in total darkness. I started back, and stumbling against one of the loose boulders reeled against the wall and nearly fell. What was the matter with me? I could hardly stand. I felt giddy and faint, and a horrible sensation of great tightness seized me across the chest. A loud ringing noise sounded in my ears. Struggling madly for breath, and with the fear of impending death upon me, I turned and tried to run from a danger I could neither understand nor grapple with. But before I had taken two steps my legs gave way from under me, and uttering a loud cry I fell insensible to the ground.

Out of an oblivion which, for all I knew, might have lasted for moments or centuries, a dawning consciousness came to me. I knew that I was lying on hard ground; that I was absolutely incapable of realising, nor had I the slightest inclination to discover, where I was. All I wanted was to lie quite still and undisturbed. Presently I opened my eyes.

Some one was bending over me and looking into my face.

"Thank G.o.d, he is not dead," I heard in whispered tones. Then, with a flash, memory returned to me.

"What has happened?" I asked.

"You may well ask that, sir," said the Inspector gravely. "It has been touch and go with you for the last quarter of an hour; and a near thing for me too."

I sat up and looked around me. Daylight was just beginning to break, and I saw that we were at the bottom of the steps that led up to the signal-box. My teeth were chattering with the cold and I was shivering like a man with ague.

"I am better now," I said; "just give me your hand."

I took his arm, and holding the rail with the other hand staggered up into the box and sat down on the bench.

"Yes, it has been a near shave," I said; "and a big price to pay for solving a mystery."

"Do you mean to say you know what it is?" asked Henderson eagerly.

"Yes," I answered, "I think I know now; but first tell me how long was I unconscious?"

"A good bit over half an hour, sir, I should think. As soon as I heard you call out I ran down as you told me, but before I got to you I nearly fainted. I never had such a horrible sensation in my life. I felt as weak as a baby, but I just managed to seize you by the arms and drag you along the line to the steps, and that was about all I could do."

"Well, I owe you my life," I said; "just hand me that brandy flask, I shall be the better for some of its contents."

I took a long pull. Just as I was laying the flask down Henderson started from my side.

"There," he cried, "the 6.30 is coming." The electric bell at the instrument suddenly began to ring. "Ought I to let her go through, sir?"

he inquired.

"Certainly," I answered. "That is exactly what we want. Oh, she will be all right."

"No danger to her, sir?"

"None, none; let her go through."

He pulled the lever and the next moment the train tore through the cutting.

"Now I think it will be safe to go down again," I said. "I believe I shall be able to get to the bottom of this business."

Henderson stared at me aghast.

"Do you mean that you are going down again to the tunnel?" he gasped.

"Yes," I said; "give me those matches. You had better come too. I don't think there will be much danger now; and there is daylight, so we can see what we are about."

The man was very loth to obey me, but at last I managed to persuade him.

We went down the line, walking slowly, and at this moment we both felt our courage revived by a broad and cheerful ray of sunshine.

"We must advance cautiously," I said, "and be ready to run back at a moment's notice."

"G.o.d knows, sir, I think we are running a great risk," panted poor Henderson; "and if that devil or whatever else it is should happen to be about--why, daylight or no daylight----"

"Nonsense! man," I interrupted; "if we are careful, no harm will happen to us now. Ah! and here we are!" We had reached the spot where I had fallen. "Just give me a match, Henderson."

He did so, and I immediately lit the lamp. Opening the gla.s.s of the lamp, I held it close to the ground and pa.s.sed it to and fro. Suddenly the flame went out.

"Don't you understand now?" I said, looking up at the Inspector.

"No, I don't, sir," he replied with a bewildered expression.

Suddenly, before I could make an explanation, we both heard shouts from the top of the cutting, and looking up I saw Bainbridge hurrying down the path. He had come in the dog-cart to fetch us.

"Here's the mystery," I cried as he rushed up to us, "and a deadlier scheme of Dame Nature's to frighten and murder poor humanity I have never seen."

As I spoke I lit the lamp again and held it just above a tiny fissure in the rock. It was at once extinguished.

"What is it?" said Bainbridge, panting with excitement.

"Something that nearly finished _me_," I replied. "Why, this is a natural escape of choke damp. Carbonic acid gas--the deadliest gas imaginable, because it gives no warning of its presence, and it has no smell. It must have collected here during the hours of the night when no train was pa.s.sing, and gradually rising put out the signal light. The constant rushing of the trains through the cutting all day would temporarily disperse it."

As I made this explanation Bainbridge stood like one electrified, while a curious expression of mingled relief and horror swept over Henderson's face.

"An escape of carbonic acid gas is not an uncommon phenomenon in volcanic districts," I continued, "as I take this to be; but it is odd what should have started it. It has sometimes been known to follow earthquake shocks, when there is a profound disturbance of the deep strata."

"It is strange that you should have said that," said Bainbridge, when he could find his voice.

"What do you mean?"

"Why, that about the earthquake. Don't you remember, Henderson," he added, turning to the Inspector, "we had felt a slight shock all over South Wales about three weeks back?"

"Then that, I think, explains it," I said. "It is evident that Pritchard really did climb the rocks in a frantic attempt to escape from the gas and fell back on to these boulders. The other man was cut down at once, before he had time to fly."

"But what is to happen now?" asked Bainbridge. "Will it go on for ever?

How are we to stop it?"

"The fissure ought to be drenched with lime water, and then filled up; but all really depends on what is the size of the supply and also the depth. It is an extremely heavy gas, and would lie at the bottom of a cutting like water. I think there is more here just now than is good for us," I added.

"But how," continued Bainbridge, as we moved a few steps from the fatal spot, "do you account for the interval between the first death and the second?"