“Did they think, perhaps, that these were two young fellows playing the fool?” I asked.

“No man who knew the sands would do so in such a place, Dr Watson.”

“Very well.”

“They were too far away by that time for Mr Gilmore or the sexton even to tell their ages. Yet, since then, neither of my brothers has been seen. It was the following morning, after the tide turned, that two policemen went to the Old Light. A Tynemouth collier, at anchor across the water, had seen the beam of the Old Light fail an hour or so before dawn. When I came from Mablethorpe, they helped me to climb the ladder and I was able to get into the barrack-room. There lay the letter in the table drawer.”

Now that she had come to the true end of her story, there was a moment’s silence, broken by Holmes.

“And there was nothing else that you noticed when you went into the barrack-room next day?”

“Abraham’s jacket was hanging behind the door. I went through the pockets. There was a piece of a pebble in one pocket.”

“What sort of pebble?”

“I should not have bothered with it-I should not even have noticed it-except that he had folded it carefully in a piece of paper. I thought at first that the paper might have a message on it. There was none, only a pebble.”

“Where is it now?”

“I took it with me. It could not possibly be of use to the police.”

“I fear you may be in error as to that, Miss Chastelnau. Do you have it with you now?”

She reached into the pocket of her dress and took out the folded paper which, as she had said, was quite blank. I got up and stood beside Holmes as he unwrapped the pebble. Before us lay what I can only describe as a small piece of clay-coated grit or possibly a rough pebble from the shoreline. It was the size of my thumb-nail, certainly no larger.

Holmes stared at it for a moment longer and then again spoke slowly to our client, as I may now call her.

“With your permission, Miss Chastelnau, I should like to retain this item for a few hours in order to examine it. You must return to Mablethorpe tonight, I believe. We shall see you safely to King’s Cross station. You may depend upon Dr Watson and I being in Sutton Cross by noon tomorrow. I will bring the pebble with me then. I fear that I cannot assure you what the outcome of this mystery will be. However, from what you have told us, I have every confidence that the riddle of your brothers’ disappearance will be resolved within the next three days.”

“Of what possible use to you can a muddy pebble be?”

“Had it not been wrapped with such care, I should probably have thought it of no use whatever. However, such careful treatment reminds me that this is hard stone, though it came apparently from a bed of soft clay to which it did not belong. I do not call that conclusive of anything-but in the light of all the other evidence it is suggestive of something.”

2

That evening, after we had seen Miss Chastelnau safely to her train, Sherlock Holmes ate his dinner from a tray beside him on his work-table. The table’s disreputable surface was stained by hydrochloric acid and the results of numerous chemical “experiments.” Scattered upon it now lay a lens and a pair of forceps, a stained penknife in a butter-dish, and a medical scalpel. A dismembered revolver had awaited his attention for two or three weeks. Close at hand were two skulls, whose owners had been hanged for murder at Tyburn a century ago and publicly dissected before a large public audience at Surgeons Hall. These two macabre fetishes now acted as book-ends for a brief row of well-thumbed reference volumes, required for immediate purposes. My friend had exchanged his formal black coat for the familiar purple of his dressing-gown.

It was after ten o’clock and his long back was curved once again over the Chastelnau pebble, as I had better call it. He had been examining it for several minutes by the aid of a jeweller’s lens screwed into his eye. Removing this eyepiece, he straightened in his chair.

“I believe we can do better, Watson. We are no common high street supplier of watches and bijouterie.”

He had scarcely spoken a word since we had returned from escorting our visitor to King’s Cross Station and he had certainly not invited conversation in the half hour since our return. Rising from his chair, he now went across to his “natural sciences” cupboard and drew out a piece of apparatus. This was a hydroscopic balance, cased in mahogany and stamped along its base in gold, “E. Dertling, London.” He sat down and placed it in front of him.

The device resembled an open-sided box of polished wood about ten inches in height, twelve inches long and six inches deep. Within it, the pivot of a brass balance was screwed to the centre of its floor. A minute weighing pan was suspended to either side of this. From the lower edge of the box protruded a small brass knob for the alignment of the scales. This had been calibrated to calculate weights to within one milligram.

“I believe we may allow for a room temperature of sixty degrees Fahrenheit, Watson. Would that be your guess?”

This was conversation at last.

“Certainly no lower than that, with the fire glowing as it is and the curtains closed.”

Holmes took Miss Chastelnau’s pebble. With a fine brush he worked over its surface to displace any loose substance that might still have adhered to it. Then, placing it in a loop of thin wire which was suspended from the pan on the right hand of the balance, he adjusted the mechanism and noted the weight of it in air. Next, taking the pebble with a pair of tweezers, he placed a small jar of water under the right-hand scale-pan, so that when he lowered the pan the pebble was immersed. Almost as an afterthought, he dipped the slender brush into the jar and went over the stone again, apparently to dislodge any bubbles of air which might give buoyancy to so small an object.

As I watched the intensity with which my friend worked I could not help thinking that Sherlock Holmes seemed less like the great consulting detective of Baker Street than a like happy child on Christmas Morning. Perhaps there was a slighter difference between the two types than I had supposed. Now he took his brass propelling pencil and made several notes on the immaculate starch of his white shirt cuffs. At length he had his answer.

“If our estimate of the room temperature is correct, Watson-and I do not think we can be far out-the specific gravity of this mineral is registered as 3.993. I do not believe it can be andradite, for I have tried it judiciously with a penknife and that will not produce a scratch upon it. Nor, I think can it be zircon of whatever type. I therefore deduce that what we are presented with appears to be a species of corundum. Only caborundum and the diamond are harder than this. Indeed, in the scale of hardness drawn up by the admirable Professor Friedrich Mohs in 1812, only the diamond exceeds it. This cannot possibly be a diamond for its specific gravity is far too high. That I believe is as far as we can go for the present.”

Holmes had given me the opportunity I had been looking for. I had not wished to annoy him or to suggest that a piece of grit picked up from the Lincolnshire fens was unlikely to be of any value or relevance to the case whatever. However, I had been thinking wistfully of sleep. A long journey lay ahead in the morning. I yawned, stretched, made my excuses and withdrew to bed.

I suppose it was about half-past eleven when my head touched the pillow. I was woken after several hours by a dreadful screaming. It might have been a banshee-or at least the sound which I had always assumed a banshee would make. I sat up with heart pounding and, at the same time, a sense of considerable irritation.

By the time I had lit a candle, the high-pitched sound came again. It was a demented shrieking from somewhere below me. Now that I was fully awake, I recognised that whatever its origins, they were mechanical and not animal. The time by my watch was ten minutes past three in the morning. It was plain that Sherlock Holmes had not yet gone to bed.