More on the subject of Thorndyke vs. Sherlock Holmes. Thorndyke is basically the prototype of the modern scientific detective; Holmes is thoroughly 19th century, a product of the virtuoso detective tradition going back to Vidocq. The differences in their modus operandi are brought to light most vividly when they have to deduce things from very similar objects.

For instance, let's take a hat.



Here's what a hat might tell Sherlock Holmes,

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I had called upon my friend Sherlock Holmes upon the second morning after Christmas, with the intention of wishing him the compliments of the season. He was lounging upon the sofa in a purple dressing-gown, a pipe-rack within his reach upon the right, and a pile of crumpled morning papers, evidently newly studied, near at hand. Beside the couch was a wooden chair, and on the angle of the back hung a very seedy and disreputable hard-felt hat, much the worse for wear, and cracked in several places. A lens and a forceps lying upon the seat of the chair suggested that the hat had been suspended in this manner for the purpose of examination.
...
"Its owner is unknown."
...
"Did he not advertise?"
"No."
"Then, what clue could you have as to his identity?"
"Only as much as we can deduce."
"From his hat?"
"Precisely."
"But you are joking. What can you gather from this old battered felt?"
"Here is my lens. You know my methods. What can you gather yourself as to the individuality of the man who has worn this article?"
I took the tattered object in my hands and turned it over rather ruefully. It was a very ordinary black hat of the usual round shape, hard and much the worse for wear. The lining had been of red silk, but was a good deal discolored. There was no maker's name; but, as Holmes had remarked, the initials "H. B." were scrawled upon one side. It was pierced in the brim for a hat-securer, but the elastic was missing. For the rest, it was cracked, exceedingly dusty, and spotted in several places, although there seemed to have been some attempt to hide the discolored patches by smearing them with ink. "I can see nothing," said I, handing it back to my friend.
"On the contrary, Watson, you can see everything. You fail, however, to reason from what you see. You are too timid in drawing your inferences."
"Then, pray tell me what it is that you can infer from this hat?"
He picked it up and gazed at it in the peculiar introspective fashion which was characteristic of him. "It is perhaps less suggestive than it might have been," he remarked, "And yet there are a few inferences which are very distinct, and a few others which represent at least a strong balance of probability. That the man was highly intellectual is of course obvious upon the face of it, and also that he was fairly well-to-do within the last three years, although he has now fallen upon evil days. He had foresight, but has less now than formerly, pointing to a moral retrogression, which, when taken with the decline of his fortunes, seems to indicate some evil influence, probably drink, at work upon him. This may account also for the obvious fact that his wife has ceased to love him."
"My dear Holmes!"
"He has, however, retained some degree of self-respect," he continued, disregarding my remonstrance. "He is a man who leads a sedentary life, goes out little, is out of training entirely, is middle-aged, has grizzled hair which he has had cut within the last few days, and which he anoints with lime-cream. These are the more patent facts which are to be deduced from his hat. Also, by the way, that it is extremely improbable that he has gas laid on in his house."
"You are certainly joking, Holmes."
"Not in the least. Is it possible that even now, when I give you these results, you are unable to see how they are attained?"
"I have no doubt that I am very stupid, but I must confess that I am unable to follow you. For example, how did you deduce that this man was intellectual?"
For answer Holmes clapped the hat upon his head. It came right over the forehead and settled upon the bridge of his nose. "It is a question of cubic capacity," said he; "A man with so large a brain must have something in it."
"The decline of his fortunes, then?"
"This hat is three years old. These flat brims curled at the edge came in then. It is a hat of the very best quality. Look at the band of ribbed silk and the excellent lining. If this man could afford to buy so expensive a hat three years ago, and has had no hat since, then he has assuredly gone down in the world."
"Well, that is clear enough, certainly. But how about the foresight and the moral retrogression?"
Sherlock Holmes laughed. "Here is the foresight," said he putting his finger upon the little disc and loop of the hat-securer. "They are never sold upon hats. If this man ordered one, it is a sign of a certain amount of foresight, since he went out of his way to take this precaution against the wind. But since we see that he has broken the elastic and has not troubled to replace it, it is obvious that he has less foresight now than formerly, which is a distinct proof of a weakening nature. On the other hand, he has endeavored to conceal some of these stains upon the felt by daubing them with ink, which is a sign that he has not entirely lost his self-respect."
"Your reasoning is certainly plausible."
"The further points, that he is middle-aged, that his hair is grizzled, that it has been recently cut, and that he uses limecream, are all to be gathered from a close examination of the lower part of the lining. The lens discloses a large number of hair-ends, clean cut by the scissors of the barber. They all appear to be adhesive, and there is a distinct odour of lime-cream. This dust, you will observe, is not the gritty, gray dust of the street but the fluffy brown dust of the house, showing that it has been hung up indoors most of the time, while the marks of moisture upon the inside are proof positive that the wearer perspired very freely, and could therefore, hardly be in the best of training."
"But his wife—you said that she had ceased to love him."
"This hat has not been brushed for weeks. When I see you, my dear Watson, with a week's accumulation of dust upon your hat, and when your wife allows you to go out in such a state, I shall fear that you also have been unfortunate enough to lose your wife's affection."
"But he might be a bachelor."
"Nay, he was bringing home the goose as a peace-offering to his wife. Remember the card upon the bird's leg."
"You have an answer to everything. But how on earth do you deduce that the gas is not laid on in his house?"
"One tallow stain, or even two, might come by chance; but when I see no less than five, I think that there can be little doubt that the individual must be brought into frequent contact with burning tallow—walks upstairs at night probably with his hat in one hand and a guttering candle in the other. Anyhow, he never got tallow-stains from a gas-jet. Are you satisfied?"

(from "The Adventure of the Blue Carbuncle")
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And now let's see what a hat might tell Thorndyke. (Notice the direct dig at Holmes in the very first lines.)

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He opened the bandbox with a flourish, and brought forth a rather shabby billycock hat.

"I understand," said he, "that by examining a hat it is possible to deduce from it, not only the bodily characteristics of the wearer, but also his mental and moral qualities, his state of health, his pecuniary position, his past history, and even his domestic relations and the peculiarities of his place of abode. Am I right in this supposition?"

The ghost of a smile flitted across Thorndyke's face as he laid the hat upon the remains of the newspaper. "We must not expect too much," he observed. "Hats, as you know, have a way of changing owners. Your own hat, for instance" (a very spruce, hard felt), "is a new one, I think."

"Got it last week," said Mr. Löwe.

"Exactly. It is an expensive hat, by Lincoln and Bennett, and I see you have judiciously written your name in indelible marking-ink on the lining. Now, a new hat suggests a discarded predecessor. What do you do with your old hats?"

"My man has them, but they don't fit him. I suppose he sells them or gives them away."

"Very well. Now, a good hat like yours has a long life, and remains serviceable long after it has become shabby; and the probability is that many of your hats pass from owner to owner; from you to the shabby-genteel, and from them to the shabby ungenteel. And it is a fair assumption that there are, at this moment, an appreciable number of tramps and casuals wearing hats by Lincoln and Bennett, marked in indelible ink with the name S. Löwe; and anyone who should examine those hats, as you suggest, might draw some very misleading deductions as to the personal habits of S. Löwe."

Mr. Marchmont chuckled audibly, and then, remembering the gravity of the occasion, suddenly became portentously solemn.

"So you think that the hat is of no use, after all?" said Mr. Löwe, in a tone of deep disappointment.

"I won't say that," replied Thorndyke. "We may learn something from it. Leave it with me, at any rate; but you must let the police know that I have it. They will want to see it, of course."

"And you will try to get those things, won't you?" pleaded Löwe.

"I will think over the case. But you understand, or Mr. Marchmont does, that this is hardly in my province. I am a medical jurist, and this is not a medico-legal case."

"Just what I told him," said Marchmont. "But you will do me a great kindness if you will look into the matter. Make it a medico-legal case," he added persuasively.

Thorndyke repeated his promise, and the two men took their departure.

For some time after they had left, my colleague remained silent, regarding the hat with a quizzical smile. "It is like a game of forfeits," he remarked at length, "and we have to find the owner of 'this very pretty thing.'" He lifted it with a pair of forceps into a better light, and began to look at it more closely.

"Perhaps," said he, "we have done Mr. Löwe an injustice, after all. This is certainly a very remarkable hat."

"It is as round as a basin," I exclaimed. "Why, the fellow's head must have been turned in a lathe!"

Thorndyke laughed. "The point," said he, "is this. This is a hard hat, and so must have fitted fairly, or it could not have been worn; and it was a cheap hat, and so was not made to measure. But a man with a head that shape has got to come to a clear understanding with his hat. No ordinary hat would go on at all.

"Now, you see what he has done—no doubt on the advice of some friendly hatter. He has bought a hat of a suitable size, and he has made it hot—probably steamed it. Then he has jammed it, while still hot and soft, on to his head, and allowed it to cool and set before removing it. That is evident from the distortion of the brim. The important corollary is, that this hat fits his head exactly—is, in fact, a perfect mould of it; and this fact, together with the cheap quality of the hat, furnishes the further corollary that it has probably only had a single owner.

"And now let us turn it over and look at the outside. You notice at once the absence of old dust. Allowing for the circumstance that it had been out all night, it is decidedly clean. Its owner has been in the habit of brushing it, and is therefore presumably a decent, orderly man. But if you look at it in a good light, you see a kind of bloom on the felt, and through this lens you can make out particles of a fine white powder which has worked into the surface."

He handed me his lens, through which I could distinctly see the particles to which he referred.

"Then," he continued, "under the curl of the brim and in the folds of the hatband, where the brush has not been able to reach it, the powder has collected quite thickly, and we can see that it is a very fine powder, and very white, like flour. What do you make of that?"

"I should say that it is connected with some industry. He may be engaged in some factory or works, or, at any rate, may live near a factory, and have to pass it frequently."

"Yes; and I think we can distinguish between the two possibilities. For, if he only passes the factory, the dust will be on the outside of the hat only; the inside will be protected by his head. But if he is engaged in the works, the dust will be inside, too, as the hat will hang on a peg in the dust-laden atmosphere, and his head will also be powdered, and so convey the dust to the inside."

He turned the hat over once more, and as I brought the powerful lens to bear upon the dark lining, I could clearly distinguish a number of white particles in the interstices of the fabric.

"The powder is on the inside, too," I said.

He took the lens from me, and, having verified my statement, proceeded with the examination. "You notice," he said, "that the leather head-lining is stained with grease, and this staining is more pronounced at the sides and back. His hair, therefore, is naturally greasy, or he greases it artificially; for if the staining were caused by perspiration, it would be most marked opposite the forehead."

He peered anxiously into the interior of the hat, and eventually turned down the head-lining; and immediately there broke out upon his face a gleam of satisfaction.

"Ha!" he exclaimed. "This is a stroke of luck. I was afraid our neat and orderly friend had defeated us with his brush. Pass me the small dissecting forceps, Jervis."

I handed him the instrument, and he proceeded to pick out daintily from the space behind the head-lining some half a dozen short pieces of hair, which he laid, with infinite tenderness, on a sheet of white paper.

"There are several more on the other side," I said, pointing them out to him.

"Yes, but we must leave some for the police," he answered, with a smile. "They must have the same chance as ourselves, you know."

"But surely," I said, as I bent down over the paper, "these are pieces of horsehair!"

"I think not," he replied; "but the microscope will show. At any rate, this is the kind of hair I should expect to find with a head of that shape."

"Well, it is extraordinarily coarse," said I, "and two of the hairs are nearly white."

"Yes; black hairs beginning to turn grey. And now, as our preliminary survey has given such encouraging results, we will proceed to more exact methods; and we must waste no time, for we shall have the police here presently to rob us of our treasure."

He folded up carefully the paper containing the hairs, and taking the hat in both hands, as though it were some sacred vessel, ascended with me to the laboratory on the next floor.

"Now, Polton," he said to his laboratory assistant, "we have here a specimen for examination, and time is precious. First of all, we want your patent dust-extractor."

The little man bustled to a cupboard and brought forth a singular appliance, of his own manufacture, somewhat like a miniature vacuum cleaner. It had been made from a bicycle foot-pump, by reversing the piston-valve, and was fitted with a glass nozzle and a small detachable glass receiver for collecting the dust, at the end of a flexible metal tube.

"We will sample the dust from the outside first," said Thorndyke, laying the hat upon the work-bench. "Are you ready, Polton?"

The assistant slipped his foot into the stirrup of the pump and worked the handle vigorously, while Thorndyke drew the glass nozzle slowly along the hat-brim under the curled edge. And as the nozzle passed along, the white coating vanished as if by magic, leaving the felt absolutely clean and black, and simultaneously the glass receiver became clouded over with a white deposit.

"We will leave the other side for the police," said Thorndyke, and as Polton ceased pumping he detached the receiver, and laid it on a sheet of paper, on which he wrote in pencil, "Outside," and covered it with a small bell-glass. A fresh receiver having been fitted on, the nozzle was now drawn over the silk lining of the hat, and then through the space behind the leather head-lining on one side; and now the dust that collected in the receiver was much of the usual grey colour and fluffy texture, and included two more hairs.

"And now," said Thorndyke, when the second receiver had been detached and set aside, "we want a mould of the inside of the hat, and we must make it by the quickest method; there is no time to make a paper mould. It is a most astonishing head," he added, reaching down from a nail a pair of large callipers, which he applied to the inside of the hat; "six inches and nine-tenths long by six and six-tenths broad, which gives us"—he made a rapid calculation on a scrap of paper—"the extraordinarily high cephalic index of 95·6."

Polton now took possession of the hat, and, having stuck a band of wet tissue-paper round the inside, mixed a small bowl of plaster-of-Paris, and very dexterously ran a stream of the thick liquid on to the tissue-paper, where it quickly solidified. A second and third application resulted in a broad ring of solid plaster an inch thick, forming a perfect mould of the inside of the hat, and in a few minutes the slight contraction of the plaster in setting rendered the mould sufficiently loose to allow of its being slipped out on to a board to dry.

We were none too soon, for even as Polton was removing the mould, the electric bell, which I had switched on to the laboratory, announced a visitor, and when I went down I found a police-sergeant waiting with a note from Superintendent Miller, requesting the immediate transfer of the hat.

"The next thing to be done," said Thorndyke, when the sergeant had departed with the bandbox, "is to measure the thickness of the hairs, and make a transverse section of one, and examine the dust. The section we will leave to Polton—as time is an object, Polton, you had better imbed the hair in thick gum and freeze it hard on the microtome, and be very careful to cut the section at right angles to the length of the hair—meanwhile, we will get to work with the microscope."

The hairs proved on measurement to have the surprisingly large diameter of 1/135 of an inch—fully double that of ordinary hairs, although they were unquestionably human. As to the white dust, it presented a problem that even Thorndyke was unable to solve. The application of reagents showed it to be carbonate of lime, but its source for a time remained a mystery.

"The larger particles," said Thorndyke, with his eye applied to the microscope, "appear to be transparent, crystalline, and distinctly laminated in structure. It is not chalk, it is not whiting, it is not any kind of cement. What can it be?"

"Could it be any kind of shell?" I suggested. "For instance—"

"Of course!" he exclaimed, starting up; "you have hit it, Jervis, as you always do. It must be mother-of-pearl. Polton, give me a pearl shirt-button out of your oddments box."

The button was duly produced by the thrifty Polton, dropped into an agate mortar, and speedily reduced to powder, a tiny pinch of which Thorndyke placed under the microscope.

"This powder," said he, "is, naturally, much coarser than our specimen, but the identity of character is unmistakable. Jervis, you are a treasure. Just look at it."

I glanced down the microscope, and then pulled out my watch. "Yes," I said, "there is no doubt about it, I think; but I must be off. Anstey urged me to be in court by 11.30 at the latest."

With infinite reluctance I collected my notes and papers and departed, leaving Thorndyke diligently copying addresses out of the Post Office Directory.

My business at the court detained me the whole of the day, and it was near upon dinner-time when I reached our chambers. Thorndyke had not yet come in, but he arrived half an hour later, tired and hungry, and not very communicative.

"What have I done?" he repeated, in answer to my inquiries. "I have walked miles of dirty pavement, and I have visited every pearl-shell cutter's in London, with one exception, and I have not found what I was looking for. The one mother-of-pearl factory that remains, however, is the most likely, and I propose to look in there to-morrow morning. Meanwhile, we have completed our data, with Polton's assistance. Here is a tracing of our friend's skull taken from the mould; you see it is an extreme type of brachycephalic skull, and markedly unsymmetrical. Here is a transverse section of his hair, which is quite circular—unlike yours or mine, which would be oval. We have the mother-of-pearl dust from the outside of the hat, and from the inside similar dust mixed with various fibres and a few granules of rice starch. Those are our data."

"Supposing the hat should not be that of the burglar after all?" I suggested.

"That would be annoying. But I think it is his, and I think I can guess at the nature of the art treasures that were stolen."

"And you don't intend to enlighten me?"

"My dear fellow," he replied, "you have all the data. Enlighten yourself by the exercise of your own brilliant faculties. Don't give way to mental indolence."

I endeavoured, from the facts in my possession, to construct the personality of the mysterious burglar, and failed utterly; nor was I more successful in my endeavour to guess at the nature of the stolen property; and it was not until the following morning, when we had set out on our quest and were approaching Limehouse, that Thorndyke would revert to the subject.

"We are now," he said, "going to the factory of Badcomb and Martin, shell importers and cutters, in the West India Dock Road. If I don't find my man there, I shall hand the facts over to the police, and waste no more time over the case."

"What is your man like?" I asked.

"I am looking for an elderly Japanese, wearing a new hat or, more probably, a cap, and having a bruise on his right cheek or temple. I am also looking for a cab-yard; but here we are at the works, and as it is now close on the dinner-hour, we will wait and see the hands come out before making any inquiries."

(from "The Anthropologist at Large")