understand how boilers burst, how brokers rig the market, how dealers fake, how jockeys pull a horse, how the poacher poaches, how the man in the dock shams catalepsy, what instruments the thief uses, what superstitions are peculiar to criminals, and what are the tools, processes, and habits of all classes of workmen. He ought to be something of a draughtsman, photographer, modeller, moulder, and stereotyper; he should know what goes to the action of walking, and what is at the back of a writing in cipher.

This type of officer does not come to the scene of action with a ready-made hypothesis. The refuge of the preconceived idea is for the tyro, or the worn-out plodder who is studiously befooled in Gaboriau. The man with the genius for his task starts out simply with his "who, what, where, with, why, how, when?"

What was the crime, who did it, when was it done, and where?

How done, and with what motive, who in the deed did share?

He has got together with all speed and prudence his first gleanings of information. Arrived at the scene of offence-a case of murder, of burglary, of arson, it matters not what-he sets out to describe it; being scrupulous not to shift the position of any object, not even to touch it, till he has got it down in his notes. He has to describe the place itself, and then (if he can do so)

the direction from which the criminal came, and the direction in which he fled; and the points from which witnesses saw, or could have seen, anything of the affair. Descriptions are, or should be, as minute as this:

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Quite near the corpse, an inch from the left hand, a red cloth rolled up in a ball, apparently of cotton and about the size of a pocket-handkerchief, one corner sticking out, lying on the ground in the direction of the head of the body."

Is there a drop of blood upon the wall? Dr. Gross holds a plumb-line at the point where this stain is found, and makes a mark where the line touches the ground.

Then follows the search for any and every kind of hidden object; and this is perhaps as excellent a touchstone as could be devised for the investigating officer. The chimney and the hole beneath the hearthstone are a little played out as receptacles. A will might be concealed in a keyhole; and in a pot bubbling with the soup for dinner eight-and-twenty gold pieces were discovered.

Traces of blood are another test of true intelligence in search. It is a rash person who will assert offhand that this or that blotch is or is not a blood-stain. Exposed to the sun, a bloodspot will very soon turn to fawn-grey. In whatever place traces of blood are suspected,

due note must be taken of the background, for blood is a very chameleon in the colours it will wear. But anything that resembles a bloodmark on the scene of crime is to be culled with more solicitude than the gardener uses in detaching blooms for the flower-show-and straight to the analyst it must go.

Then there is the perquisition for blood which the criminal has striven to mask or purge away. Science, personified by the investigating officer, seeks the scent and coldly follows it. You have in ward a man arrested on the charge of murder. If he is the murderer, and has had the chance of washing, he has washed. But blood is a most dread tale-bearer. It will lurk all but unseen beneath the nails, or under the skin where the nails and fingers meet. A magnifying glass will summon upon the cheek or the back of the hand a faint avowal of the blood that has but just been cleansed from it. So also will your expert chase the red element that a hasty scouring has driven into the linings of a waistcoat or the chinks of a floor. No sapient criminal would use warm water to get rid of blood, yet five criminals in six think it twice as efficacious as the running stream. A little more familiarity with the powers both of nature and of science, and the murderer would be less apt to court the

noose.

Finger-prints in blood will give us very start

ling dramas. Wonderful indeed are the revelations brought about by those slender patterns on the finger-tips-wherein lie, thanks mainly to Sir Edward Henry, our very newest (and in some respects most certain) proof of human identity. It may be said with all brevity that these prints, securely taken, are read as easily as possible (they can, of course, be magnified in any degree), and that the chance that two finger-prints of different individuals will agree with one another is smaller than one in sixty-four thousand millions.

III

When a difficult quest has advanced to a certain stage, the investigating officer has recourse to the expert. The expert is, in fact, his chief auxiliary. In these two we have the agents most directly concerned in criminal investigation; the official person deputed to act for justice, and the expert-in whatever department-who stands to him in the relation of principal assistant. Upon these two, each understanding his work and enthusiastic in it, and the pair of them harmoniously collaborating, we mainly rely for proof of the graver crimes.

It is the part of the investigator to know the precise moment at which he should call to his

aid the man with the special information, and the questions proper to submit to him.

An indispensable expert in sundry cases is the medical jurisprudent. This is our great authority in a matter of post-mortem wounds, of illnesses, etc. The medical expert who is thoroughly abreast of his times will reconstruct for us a face so defeatured by decay that

neither friend nor relative could claim it. I have not space to detail the process, which begins with cutting off the head and taking out the brain. In a puzzling problem of the mental affections you will naturally turn to your medical jurisprudent.

The expert dentist is sometimes another very useful person. A banker was murdered in St. Petersburg. Near the body was found a cigarholder with an amber mouthpiece. On the holder, which was of such a shape that it could be held in the mouth in one position only, were two teeth marks. These marks, closely scrutinised, were seen to have been made by two teeth of unequal length. The teeth of the murdered man were not irregular in this way. Those of his nephew were; and, certain other facts coming to light, the nephew was arrested.

We are punctilious nowadays in taking the finger-prints of a live criminal or suspected person. Dr. Gross shall show us that it may be quite necessary, to the ends of justice, to