Half-past nine on the following morning found us spinning along the Albert Embankment in a hansom to the pleasant tinkle of the horse's bell. Thorndyke appeared to be in high spirits, though the full enjoyment of the matutinal pipe precluded fluent conversation. As a precaution, he had put my notebook in his pocket before starting, and once or twice he took it out and looked over its pages; but he made no reference to the object of our quest, and the few remarks that he uttered would have indicated that his thoughts were occupied with other matters.  
Arrived at Vauxhall Station, we alighted and forthwith made our way to the bridge that spans Upper Kennington Lane near its junction with Harleyford Road.
 
"Here is our starting point," said Thorndyke. "From this place to the house is about three hundred yards—say four hundred and twenty paces—and at about two hundred paces we ought to reach our patch of new road-metal. Now, are you ready? If we keep step we shall average our stride."
 
We started together at a good pace, stepping out with military regularity and counting aloud as we went. As we told out the hundred and ninety-fourth pace I observed Thorndyke nod towards the roadway a little ahead, and, looking at it attentively as we approached, it was easy to see by the regularity of surface and lighter colour, that it had recently been re-metalled.
 
Having counted out the four hundred and twenty paces, we halted, and Thorndyke turned to me with a smile of triumph.
 
"Not a bad estimate, Jervis," said he. "That will be your house if I am not much mistaken. There is no other mews or private roadway in sight."
 
He pointed to a narrow turning some dozen yards ahead, apparently the entrance to a mews or yard and closed by a pair of massive wooden gates.
 
"Yes," I answered, "there can be no doubt that this is the place; but, by Jove!" I added, as we drew nearer, "the nest is empty! Do you see?"
 
I pointed to a bill that was stuck on the gate, bearing, as I could see at this distance, the inscription "To Let."
 
"Here is a new and startling, if not altogether unexpected, development," said Thorndyke, as we stood gazing at the bill; which set forth that "these premises, including stabling and workshops," were "to be let on lease or otherwise," and referred inquiries to Messrs. Ryebody Brothers, house-agents and valuers, Upper Kennington Lane. "The question is, should we make a few inquiries of the agent, or should we get the keys and have a look at the inside of the house? I am inclined to do both, and the latter first, if Messrs. Ryebody Brothers will trust us with the keys."
 
We proceeded up the lane to the address given, and, entering the office, Thorndyke made his request—somewhat to the surprise of the clerk; for Thorndyke was not quite the kind of person whom one naturally associates with stabling and workshops. However, there was no difficulty, but as the clerk sorted out the keys from a bunch hanging from a hook, he remarked:
 
"I expect you will find the place in a rather dirty and neglected condition. The house has not been cleaned yet; it is just as it was left when the brokers took away the furniture."
 
"Was the last tenant sold up, then?" Thorndyke asked.
 
"Oh, no. He had to leave rather unexpectedly to take up some business in Germany."
 
"I hope he paid his rent," said Thorndyke.
 
"Oh, yes. Trust us for that. But I should say that Mr. Weiss—that was his name—was a man of some means. He seemed to have plenty of money, though he always paid in notes. I don't fancy he had a banking account in this country. He hadn't been here more than about six or seven months and I imagine he didn't know many people in England, as he paid us a cash deposit in lieu of references when he first came."
 
"I think you said his name was Weiss. It wouldn't be H. Weiss by any chance?"
 
"I believe it was. But I can soon tell you." He opened a drawer and consulted what looked like a book of receipt forms. "Yes; H Weiss. Do you know him, sir?"
 
"I knew a Mr. H. Weiss some years ago. He came from Bremen, I remember."
 
"This Mr. Weiss has gone back to Hamburg," the clerk observed.
 
"Ah," said Thorndyke, "then it would seem not to be the same. My acquaintance was a fair man with a beard and a decidedly red nose and he wore spectacles."
 
"That's the man. You've described him exactly," said the clerk, who was apparently rather easily satisfied in the matter of description.
 
"Dear me," said Thorndyke; "what a small world it is. Do you happen to have a note of his address in Hamburg?"
 
"I haven't," the clerk replied. "You see we've done with him, having got the rent, though the house is not actually surrendered yet. Mr Weiss's housekeeper still has the front-door key. She doesn't start for Hamburg for a week or so, and meanwhile she keeps the key so that she can call every day and see if there are any letters."
 
"Indeed," said Thorndyke. "I wonder if he still has the same housekeeper."
 
"This lady is a German," replied the clerk, "with a regular jaw-twisting name. Sounded like Shallybang."
 
"Schallibaum. That is the lady. A fair woman with hardly any eyebrows and a pronounced cast in the left eye."
 
"Now that's very curious, sir," said the clerk. "It's the same name, and this is a fair woman with remarkably thin eyebrows, I remember, now that you mention it. But it can't be the same person. I have only seen her a few times and then only just for a minute or so; but I'm quite certain she had no cast in her eye. So, you see, sir, she can't be the same person. You can dye your hair or you can wear a wig or you can paint your face; but a squint is a squint. There's no faking a swivel eye."
 
Thorndyke laughed softly. "I suppose not; unless, perhaps, some one might invent an adjustable glass eye. Are these the keys?"
 
"Yes, sir. The large one belongs to the wicket in the front gate. The other is the latch-key belonging to the side door. Mrs. Shallybang has the key of the front door."
 
"Thank you," said Thorndyke. He took the keys, to which a wooden label was attached, and we made our way back towards the house of mystery, discussing the clerk's statements as we went.
 
"A very communicable young gentleman, that," Thorndyke remarked. "He seemed quite pleased to relieve the monotony of office work with a little conversation. And I am sure I was very delighted to indulge him."
 
"He hadn't much to tell, all the same," said I.
 
Thorndyke looked at me in surprise. "I don't know what you would have, Jervis, unless you expect casual strangers to present you with a ready-made body of evidence, fully classified, with all the inferences and implications stated. It seemed to me that he was a highly instructive young man."
 
"What did you learn from him?" I asked.
 
"Oh, come, Jervis," he protested; "is that a fair question, under our present arrangement? However, I will mention a few points. We learn that about six or seven months ago, Mr. H. Weiss dropped from the clouds into Kennington Lane and that he has now ascended from Kennington Lane into the clouds. That is a useful piece of information. Then we learn that Mrs. Schallibaum has remained in England; which might be of little importance if it were not for a very interesting corollary that it suggests."
 
"What is that?"
 
"I must leave you to consider the facts at your leisure; but you will have noticed the ostensible reason for her remaining behind. She is engaged in puttying up the one gaping joint in their armour. One of them has been indiscreet enough to give this address to some correspondent—probably a foreign correspondent. Now, as they obviously wish to leave no tracks, they cannot give their new address to the Post Office to have their letters forwarded, and, on the other hand, a letter left in the box might establish such a connection as would enable them to be traced. Moreover, the letter might be of a kind that they would not wish to fall into the wrong hands. They would not have given this address excepting under some peculiar circumstances."
 
"No, I should think not, if they took this house for the express purpose of committing a crime in it."
 
"Exactly. And then there is one other fact that you may have gathered from our young friend's remarks."
 
"What is that?"
 
"That a controllable squint is a very valuable asset to a person who wishes to avoid identification."
 
"Yes, I did note that. The fellow seemed to think that it was absolutely conclusive."
 
"And so would most people; especially in the case of a squint of that kind. We can all squint towards our noses, but no normal person can turn his eyes away from one another. My impression is that the presence or absence, as the case might be, of a divergent squint would be accepted as absolute disproof of identity. But here we are."
 
He inserted the key into the wicket of the large gate, and, when we had stepped through into the covered way, he locked it from the inside.
 
"Why have you locked us in?" I asked, seeing that the wicket had a latch.
 
"Because," he replied, "if we now hear any one on the premises we shall know who it is. Only one person besides ourselves has a key."
 
His reply startled me somewhat. I stopped and looked at him.
 
"That is a quaint situation, Thorndyke. I hadn't thought of it. Why she may actually come to the house while we are here; in fact, she may be in the house at this moment."
 
"I hope not," said he. "We don't particularly want Mr. Weiss to be put on his guard, for I take it, he is a pretty wide-awake gentleman under any circumstances. If she does come, we had better keep out of sight. I think we will look over the house first. That is of the most interest to us. If the lady does happen to come while we are here, she may stay to show us over the place and keep an eye on us. So we will leave the stables to the last."
 
We walked down the entry to the side door at which I had been admitted by Mrs. Schallibaum on the occasion of my previous visits. Thorndyke inserted the latch-key, and, as soon as we were inside, shut the door and walked quickly through into the hall, whither I followed him. He made straight for the front door, where, having slipped up the catch of the lock, he began very attentively to examine the letter-box. It was a somewhat massive wooden box, fitted with a lock of good quality and furnished with a wire grille through which one could inspect the interior.
 
"We are in luck, Jervis," Thorndyke remarked. "Our visit has been most happily timed. There is a letter in the box."
 
"Well," I said, "we can't get it out; and if we could, it would be hardly justifiable."
 
"I don't know," he replied, "that I am prepared to assent off-hand to either of those propositions; but I would rather not tamper with another person's letter, even if that person should happen to be a murderer. Perhaps we can get the information we want from the outside of the envelope."
 
He produced from his pocket a little electric lamp fitted with a bull's-eye, and, pressing the button, threw a beam of light in through the grille. The letter was lying on the bottom of the box face upwards, so that the address could easily be read.
 
"Herrn Dr. H. Weiss," Thorndyke read aloud. "German stamp, postmark apparently Darmstadt. You notice that the 'Herrn Dr.' is printed and the rest written. What do you make of that?"
 
"I don't quite know. Do you think he is really a medical man?"
 
"Perhaps we had better finish our investigation, in case we are disturbed, and discuss the bearings of the facts afterwards. The name of the sender may be on the flap of the envelope. If it is not, I shall pick the lock and take out the letter. Have you got a probe about you?"
 
"Yes; by force of habit I am still carrying my pocket case."
 
I took the little case from my pocket and extracting from it a jointed probe of thickish silver wire, screwed the two halves together and handed the completed instrument to Thorndyke; who passed the slender rod through the grille and adroitly turned the letter over.
 
"Ha!" he exclaimed with deep satisfaction, as the light fell on the reverse of the envelope, "we are saved from the necessity of theft—or rather, unauthorized borrowing—'Johann Schnitzler, Darmstadt.' That is all that we actually want. The German police can do the rest if necessary."
 
He handed me back my probe, pocketed his lamp, released the catch of the lock on the door, and turned away along the dark, musty-smelling hall.
 
"Do you happen to know the name of Johann Schnitzler?" he asked.
 
I replied that I had no recollection of ever having heard the name before.
 
"Neither have I," said he; "but I think we may form a pretty shrewd guess as to his avocation. As you saw, the words 'Herrn Dr.' were printed on the envelope, leaving the rest of the address to be written by hand. The plain inference is that he is a person who habitually addresses letters to medical men, and as the style of the envelope and the lettering—which is printed, not embossed—is commercial, we may assume that he is engaged in some sort of trade. Now, what is a likely trade?"
 
"He might be an instrument maker or a drug manufacturer; more probably the latter, as there is an extensive drug and chemical industry in Germany, and as Mr. Weiss seemed to have more use for drugs than instruments."
 
"Yes, I think you are right; but we will look him up when we get home. And now we had better take a glance at the bedroom; that is, if you can remember which room it was."
 
"It was on the first floor," said I, "and the door by which I entered was just at the head of the stairs."
 
We ascended the two flights, and, as we reached the landing, I halted.
 
"This was the door," I said, and was about to turn the handle when Thorndyke caught me by the arm.
 
"One moment, Jervis," said he. "What do you make of this?"
 
He pointed to a spot near the bottom of the door where, on close inspection, four good-sized screw-holes were distinguishable. They had been neatly stopped with putty and covered with knotting, and were so nearly the colour of the grained and varnished woodwork as to be hardly visible.
 
"Evidently," I answered, "there has been a bolt there, though it seems a queer place to fix one."
 
"Not at all," replied Thorndyke. "If you look up you will see that there was another at the top of the door, and, as the lock is in the middle, they must have been highly effective. But there are one or two other points that strike one. First, you will notice that the bolts have been fixed on quite recently, for the paint that they covered is of the same grimy tint as that on the rest of the door. Next, they have been taken off, which, seeing that they could hardly have been worth the trouble of removal, seems to suggest that the person who fixed them considered that their presence might appear remarkable, while the screw-holes, which have been so skilfully and carefully stopped, would be less conspicuous.
 
"Then............