because the writer was unable to find the word more than once in
the book on which his cryptogram was based.

"Having once arrived at the conclusion that some book had been used
as the basis of the cryptogram, my next supposition that each group
of three sets of numbers showed the page of the book, the number of
the line from the top, and the position of the required word in
that line, seemed at once borne out by an analysis of the figures
themselves. Thus, taking the first set of figures in each group, I
found that in no case did they run to a higher number than 500,
which would seem to indicate that the basis-book was limited to
that number of pages. The second set of figures ran to no higher
number than 60, which would seem to limit the lines on each page to
that number. The third set of figures in no case yielded a higher
number than 12, which numerals, according to my theory, would
indicate the maximum number of words in each line. Thus you have at
once (if such information is of any use to you) a sort of a key to
the size of the required volume.

"I think I have now written enough, my dear Ducie, to afford you
some idea of the method by means of which my conclusions have been
arrived at. If you wish for further details I will supply them--but
by word of mouth, an it be all the same to your honour; for this
child detests letter-writing, and has taken a vow that if he reach
the end of his present pen-and-ink venture in safety, he will never
in time to come devote more than two pages of cream note to even
the most exacting of friends: the sequitur of which is, that if you
want to know more than is here set down you must give the writer a
call, when you shall be talked to to your heart's content.