"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. "Yes, and if there is no card there and the pins in the square box are started down, they are automatically stopped before they reach the mercury so as not to make a contact on every point. Also if a card were there without any holes punched, none of the pins would reach the mercury and no contact would be made." "But with a punched census card," interrupted the boy, eager to show that he understood, "the pins go through the holes in the cards and do not go through where no holes are punched, so that somehow the number of holes in the card is registered. But still, there's so much difference in the cards that I don't see how this machine can verify them, can tell which are right and which wrong!" "There is variety enough," answered the chief, "for of the hundred million cards punched, no two are exactly the same, they could not be." "Couldn't it happen perhaps that two people of the same age should do the same work, be both married and so forth?" asked the boy interestedly. "They would have to live in the same district, they would have to be employed the same way, they would have both to be married and have the same number of children and a whole lot more things, and even then--the cards would be different for they would represent different numbers on the schedule on which their names were registered. No, there are not two cards in the entire series punched alike." "Then I don't see how in the wide world this machine can tell which cards are right among millions so entirely different from each other." "They don't verify by finding the cards that are right," was the answer, "but by picking out the cards that are wrong." "What's the difference?" "There is a wide difference. You can see that it would be easy enough to arrange that machine so that if a wrong combination of contacts were made the bell would not ring. Such wiring might be highly complex, but you see the idea is simple. For a right group of contacts, all the wires are satisfied, as it were, and the bell rings; for an error, one wire, cut in on by a wrong wire, breaks the contact, and the bell does not ring." "But what do you mean by a wrong grouping?" asked the boy. "You ought to be able to guess that," the chief said reproachfully. "For instance if a card is punched 'Wf' for Wife and also is punched 'Male' that card is sure to be wrong, and if 'Emp' for employer is punched on the same card as an age punch showing the person to be a three-year-old youngster, the card is wrong. There are twenty-three different possibilities of error which are checked by this verification machine, and for any one of these twenty-three reasons a card is thrown out." "For example if 'Na' for naturalized is punched on the same card as 'N' for native-born, and things of that sort, I suppose?" the boy questioned.