arked. "I am not an
expert in photography, Paul, and I'd like a full explanation. And make
it as simple as you can."

"It's a very simple thing indeed," said the son. "One day while I was
wondering how we could best catch the man who was getting at the books,
that clock happened to strike, and somehow it reminded me that in our
photographic society at college we had once suggested that it would be
amusing to attach a detective camera to a timepiece and take snapshots
every few minutes all through the day. I saw that this clock of ours
faced the safe, and that it couldn't be better placed for the purpose.
So when I had thought out my plan, I came over here and pretended that
the clock was wrong, and in setting it right I broke off the
minute-hand. Then I had a man I know send for it for repairs; he is
both an electrician and an expert photographer. Together we worked out
this device. Here is a small snap-shot camera loaded with a hundred and
fifty films; and here is the electrical attachment which connects with
the clock so as to take a photograph every ten minutes from eight in
the morning to six at night. We arranged that the magnet should turn
the spool of film after every snap-shot."

"Well!" cried Mr. Wheatcroft. "I don't know much about these things,
but I read the papers, and I suppose you mean that the clock 'pressed
the button,' and the electricity pulled the string."

"That's it precisely," the young man responded. "Of course I wasn't
quite sure how it would work, so I thought I would try it first on a
week-day when we were all here. It did work all right, and I made
several interesting discoveries. I found that Mike smoked a pipe in
this office--and that Bob played leap-frog in the store and stood on
his head in the corner there up against the safe!"

"The confounded young rascal!" interrupted Mr. Wheatcroft.

Paul smiled as he continued. "I found also that Mr. Wheatcroft was
captivated by a pretty book-agent and bought two bulky volumes he
didn't want."

Mr. Wheatcroft looked sheepish for a moment.

"Oh, that's how you knew, is it?" he growled, running his hands
impatiently through his shock of hair.

"That's how I knew," Paul replied. "I told you I had an eye on you. It
was the lone eye of the camera. And on Sunday it kept watch for us
here, winking every ten minutes. From eight o'clock in the morning to
three in the afternoon it winked forty-two times, and all it saw was
the same scene, the empty