"Were you ever handed a bulky book and were surprised to find it extremely light?" said he to Fuller. "That oddity of thickness combined with lightness applies also to this lid."
The tip of the long inquiring finger ran along the edge of the lid; the quick, observant glance followed close behind. Instantly Fuller caught the suggestion.
"That's so," said he, eagerly; "it may be hollow."
"On each side of the lock," said Ashton-Kirk, "there is an inlaid strip. Look closely and you will see slight marks at the ends of each where the point of a knife has been inserted from time to time."
As he spoke he brought his own knife into play. Out came one of the inlaid pieces, disclosing a shallow opening. But it was empty. However, the second one revealed a number of sheets of paper. With the aid of the knife blade he managed to work these out; then spreading them upon the desk the two men examined them with attention.
"Hello," said Fuller, "here is that thing which I said a while ago looked like a ground plan."
"And here are the variously colored versions of the same, just as Warwick described them," said the secret agent. "They are precisely alike, but some are in brown, others in black, still others are in red, while some again are in blue. And here are the ones done upon neutral paper, in white."
"Is it possible, do you think," questioned Fuller, "that anything was meant by the differing colors?"
"There is nothing to convince me that such is not the case," replied Ashton-Kirk. "Chance seldom rules in a matter of consequence."
"Could the change in color not be ascribed merely to the fact that the draughtsman used the one that came first to his hand?"