Betty wanted to know.

"Because, just at that point--where the description of the birth-mark,
and its location, should appear--the letter is torn. A corner is gone.
I have no more idea of what sort of identifying mark my sister has, than
have you. It is worse than before, for I saw hope ahead of me, only to
see it disappear now.

"I feel sure that the girl referred to in the old letter is my sister;
but how can I identify her, in case I meet her, until I know what sort
of a mark she has, and where it is?"

"You can't!" declared Will, positively.

"And that makes it all the more tantalizing," went on Mr. Blackford.
"They even--that firm I spoke of--they even had located the part of the
country where it might be possible my sister was, and now to have it
fail this way----"

"Where did they say she might be?" asked Amy.

"Somewhere up in Canada. But it is rather vague. If only that piece was
not torn off the edge of the letter!"

"Can't you find it somewhere?" asked Mollie. "Maybe in forwarding it the
people you hired tore it by accident."

"I thought of that, so I telephoned as soon as I got this letter, asking
where the missing piece was. I got word back that they knew nothing
about it."

There was silence for a moment, while they all looked at the mutilated
document Mr. Blackford held up. It showed a tear across one corner, a
tear that disposed of the most vital piece of information contained on
the whole paper.

"That's too bad," spoke Amy, sympathetically.

"Yes," agreed Mollie, as she put back a stray and rebellious lock of
hair, "it spoils all your plans, I suppose, Mr. Blackford."

"In a way, yes. But I'm not going to give up. I'm going to find out
where they got this document from, and go there. It may have been in
some old attic trunk, among some--love letters--and the missing piece
may be there."

"Without it you're all at sea," declared Will. "You don't know what sort
of a mark to look for, nor where it might be."

"And he can't very well go around asking all the girls he meets if they
have peculiar birth-marks," commented Mollie.

"Well, I hardly know why I told you my troubles," said the young man,
"but----"

"Why shouldn't you?" asked Betty, pleasantly. "We are interested in you,
of course, ever since----"

"That five hundred dollar bill you thought was gone for good," added
Amy. "But if we hear of anything----" and she paused suggestively.

"I wish you'd let me know!" exclaimed Mr. B