It was one of the government detective's pet theories that every crime, no matter how puzzling, could be solved by application of common-sense principles and the rules of logic. "The criminal with brains," he was fond of saying, "will deliberately try to throw you off the scent. Then you've got to take your time and separate the wheat from the chaff--the false leads from the true. But the man who commits a crime on the spur of the moment--or who flatters himself that he hasn't left a single clue behind--is the one who's easy to catch. The cleverest crook in the world can't enter a room without leaving his visiting card in some way or other. It's up to you to find that card and read the name on it. And common sense is the best reading glass." Requesting that his mission be kept secret, Drummond said that he would like to examine Vault No. Six. "Let Cochrane open the vault for me and then have Jamison and Strubel open the grille," he directed. "Unless Mr. Bosbyshell opened the vault door," Preston reminded him, "there's no one but Cochrane who could do it. It won't be necessary, however, to have either of the others open the grille--the door was taken from its hinges this morning in order the better to examine the place and it hasn't yet been replaced." "All right," agreed Drummond. "Let's have Cochrane work the outer combination, then. I'll have a look at the other two later." Accompanied by the director and the superintendent, Drummond made his way to the basement where they were joined by the official weigher, a man well over fifty, who was introduced by Preston to "Mr. Drummond, a visitor who is desirous of seeing the vaults." "I understand that you are the only man who can open them," said the detective. "Suppose we look into this one," as he stopped, as if by accident, before Vault No. 6. Cochrane, without a word, bent forward and commenced to twirl the combination. A few spins to the right, a few to the left, back to the right, to the left once more--and he pulled at the heavy door expectantly. But it failed to budge. Again he bent over the combination, spinning it rapidly. Still the door refused to open. "I'm afraid I'll have to ask you to help me with this, Superintendent," Cochrane said, finally. "It doesn't seem to work, somehow." But, under Bosbyshell's manipulation, the door swung back almost instantly. "Nothing wrong with the combination," commented Preston. Drummond smiled. "Has the combination been changed recently?" he asked. "Not for the past month," Bosbyshell replied. "We usually switch all of them six times a year, just as a general precaution--but this has been the same for the past few weeks. Ever since the fifteenth of last month, to be precise." Inside the vault Drummond found that, as Preston had stated, the door to the grille had been taken from its hinges, to facilitate the work of the men who had weighed the gold, and had not been replaced.