When he was over there one day, after hours, Hamilton saw Mr. Cullern on the floor.
"Still looking for information?" questioned the older man, with a smile.
"Yes, sir," answered the boy, "I've been watching this machine and I've spoken to one or two of the operators about the principle of it, but they none of them seem to know. They knew how to run it, and that was about all."
"The principle is simple enough," the chief replied, "but it would be a bit hard to understand the combination unless you had the clew. Then it is all as clear as day, although the machine itself is a little complicated. You noticed, of course, that the operator lays a card on this plate which is full of holes, and you probably noticed that these holes correspond with the points on the card, and that the way in which the card is fed into the machine insures that the holes shall coincide exactly."
"That I saw," Hamilton answered, "and I could see, of course, that this was one of the most important parts of the machine, and that upon it a good deal of the exactness of the work depended."
[Illustration: TABULATING MACHINE. Mechanism whereby the punched cards are verified and every error prevented, and which also tabulates and numbers all records taken. (_Courtesy of the Bureau of the Census._)]
"It does," the other replied. "Now if you look into those holes in the plate you can see a little cup of bright metal under each hole. What do you suppose that is?"
"I'm not sure, of course," the boy responded, "but it looks very much like quicksilver."
"That's exactly what it is, quicksilver, or mercury. Now mercury, you ought to know, can transmit an electric current, so that if an electrically charged pin comes down into the cup of mercury, the cup itself being attached to an electric current, a circuit is formed."
"Now I'm beginning to see," the boy said, "but what is the idea of the cup of mercury; could not the pin just as well touch on a metal plate?"
"It could, of course, but a piece of dust between would prevent contact, the pins would wear away quickly, and the plate would get worn, whereas, by the pin just dropping into the mercury there is no friction and no fear of a missed contact."
"The pins are in that square box at the end of the long arm which comes down every time a card is put on the plate, aren't they, Mr. Cullern?" asked Hamilton.