Clare had been using the protractor herself, quickly familiarizing herself with it.
"Another possible divergence," went on Kennedy, "is the perpendicular position of the letter in relation to the line. That is of great value in individualizing a machine. It is very seldom that machines, even when they are new, are perfect in this particular. It does not seem much until you magnify it. Then anyone can see it, and it is a characteristic that is fixed, continuous, and not much changed by variations in speed or methods of writing.
"Here's another thing. Typewriter faces are not flat like printing type, but are concaved to conform to the curve of the printing surface of the roller. When they are properly adjusted all portions should print uniformly. But when they are slightly out of position in any direction the two curved surfaces of type and roller are not exactly parallel and therefore don't come together with uniform pressure. The result is a difference in intensity in different parts of the impression."
It was fascinating to see Craig at work over such minute points which we had never suspected in so common a thing as ordinary typewriting.
"Then you can identify these letters positively?" asked Clare.
"Positively," answered Craig. "If two machines of the same make were perfect to begin with and in perfect condition—which is never found to be the case when they are critically examined—the work from one would be theoretically indistinguishable from that of another until actual use had affected them differently. The work of any number of machines begins inevitably to diverge as soon as they are used. Since there are thousands of possible particulars in which differences may develop, it very soon becomes possible to identify positively the work of a particular typewriting machine."
"How about the operator?" I asked curiously.
"Different habits of touch, spacing, speed, arrangement, and punctuation all may also tend to show that a particular piece of writing was or was not done by one operator. In other words, typewriting individuality in many cases is of the most positive and convincing character and reaches a degree of certainty which may almost be described as absolute proof. The identification of a typewritten document in many cases is exactly parallel to the identification of an individual who precisely answers a general description as to features, complexion, size, and in addition matches a long detailed list of scars, birthmarks, deformities, and individual peculiarities."
Together we three began an exhaustive examination of the letters, and as Kennedy called off the various characteristics of each type on the standard keyboard we checked them up. It did not take long to convince us, nor would it have failed to convince the most sceptical, that both had come from the same source and the same writer.
"You see," concluded Kennedy triumphantly, "we have advanced a long step nearer the solution of at least one of the problems of this case."