ifferent persons, probably
strangers, who possess precisely the same peculiarities in penmanship.

This principle of the law of probabilities is applied in the case of the
identification of persons "wanted" by the police. For example, the
official description of an absconding forger runs as follows:--"He has a
habit of rubbing his right thumb against the middle finger as if turning
a ring. He frequently strokes his right eyebrow with right forefinger
when engaged in writing; when perplexed, he bites his lower lip and
clenches and unclenches his fingers."

Now there are, probably, thousands of people who do every one of these
things singly, but the chances are millions to one against there being
two people who do them all as described in the official placard. In like
manner there may be a multitude of writers who form an _f_ or _k_ with a
peculiar exaggerated buckle. Thousands more may make certain letters in
the same way, but to assume that there are two persons who possess
equally the whole twelve characteristics noted by the expert is to
strain coincidence to the breaking-point of absurdity.

Therefore, it follows that it is the weight of cumulative evidence of
similarity in the production of unusual tricks of style that proclaims a
common authorship for two apparently different writings.

It may be, and often is, the case that the peculiarities or tricks in
the original have been imitated in the suspected writing. As the result
of his experience in knowing what to look for in a copied document, the
expert is not deceived. However good the copy, there are always apparent
to the trained eye evidences that prove another and stranger hand, plain
as the difference between the firm, clear line of the drawing master and
the broken saw-edged effort of the pupil. Habitual observation trains
the eye to an extent that would scarcely be credited unless proved by
experiment. The art of observation cannot be taught; it must be the
outcome of practice. The most the teacher can do is to indicate the
lines on which the study should be carried out, and offer hints and
suggestions as to what to look for. The rest is in the hands of the
student.

CHAPTER II.

MEASUREMENT AND ITS APPLIANCES.

The appliances necessary for the work of examination are, a good
magnifying or reading glass of the greatest power obtainable, a pair of
fine compasses or dividers, a horn or celluloid protractor for measuring
angles of slope, and a