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.

"And many others of similar character," the older man agreed.

"But how about insufficiently punched cards?" queried Hamilton. "I can
see that it would be easy to arrange the wires so as to catch really bad
inconsistencies, but supposing a figure were only left out, there would
be no contact made to show the error."

"Except in the age column," was the reply, "there is supposed to be a
punch in every field and only one. Any field which does not have a
contact from every card registers its disapproval by throwing out that
card."

"And what happens to the rejected cards?" asked Hamilton, with interest.

"A checker-up compares them with the original schedules, and if
incorrectly punched, punches a new card, if only insufficiently punched,
punches the missing place. But the number of cards found wrong does not
reach a high percentage."

"You know I've been thinking," Hamilton said thoughtfully, "that while I
suppose it is all right getting all those holes punched in a card, and
so forth, I should think it would be fearfully hard to handle the card
afterwards. All these little holes look so much alike."

"To the eye, perhaps," the chief said, "but you must remember that these
cards are never sorted by