"But even if Cochrane or I--yes, I'm just as much to be suspected as he--could have managed to open the vault door unseen, we could not have gotten inside the iron grille which guards the gold in the interior of the vault. That is always kept locked, with a combination known to two other men only. There's too much gold in each one of these vaults to take any chance with, which is the reason for this double protection. Two men--Cochrane and I--handle the combination to the vault door and open it whenever necessary. Two others--Jamison and Strubel--are the only ones that know how to open the grille door. One of them has to be present whenever the bars are put in or taken away, for the men who can get inside the vault cannot enter the grille, and the men who can manipulate the grille door can't get into the vault." "It certainly sounds like a burglar-proof combination," commented Drummond. "Is there any possibility for conspiracy between"--and he hesitated for the fraction of a second--"between Cochrane and either of the men who can open the grille door?" "Apparently not the least in the world," replied Preston. "So far as we know they are all as honest as the day--" "But the fact remains," Drummond interrupted, "that the gold is missing." "Exactly--but the grille door was sealed with the official governmental stamp when we entered the vault yesterday. That stamp is applied only in the presence of both men who know the combination. So the conspiracy, if there be any, must have included Cochrane, Strubel, and Jamison--instead of being a two-man job." "How much gold did you say was missing?" inquired the Treasury operative, taking another tack. "Seven hundred pounds--fifty bars of fourteen pounds each," answered Bosbyshell. "That's another problem that defies explanation. How could one man carry away all that gold without being seen? He'd need a dray to cart it off, and we're very careful about what goes out of the Mint. There's a guard at the front door all the time, and no one is allowed to leave with a package of any kind until it has been examined and passed." A grunt was Drummond's only comment--and those who knew the Secret Service man best would have interpreted the sound to mean studious digestion of facts, rather than admission of even temporary defeat.