"Suppose that Cadogan West wished to make his way into the building after hours; he would need three keys, would he not, before the could reach the papers?"
"Yes, he would. The key of the outer door, the key of the office, and the key of the safe."
"Only Sir James Walter and you had those keys?"
"I had no keys of the doors--only of the safe."
"Was Sir James a man who was orderly in his habits?"
"Yes, I think he was. I know that so far as those three keys are concerned he kept them on the same ring. I have often seen them there."
"And that ring went with him to London?"
"He said so."
"And your key never left your possession?"
"Never."
"Then West, if he is the culprit, must have had a duplicate. And yet none was found upon his body. One other point: if a clerk in this office desired to sell the plans, would it not be simply to copy the plans for himself than to take the originals, as was actually done?"
"It would take considerable technical knowledge to copy the plans in an effective way."
"But I suppose either Sir James, or you, or West has that technical knowledge?"
"No doubt we had, but I beg you won't try to drag me into the matter, Mr. Holmes. What is the use of our speculating in this way when the original plans were actually found on West?"
"Well, it is certainly singular that he should run the risk of taking originals if he could safely have taken copies, which would have equally served his turn."
"Singular, no doubt--and yet he did so."
"Every inquiry in this case reveals something inexplicable. Now there are three papers still missing. They are, as I understand, the vital ones."
"Yes, that is so."
"Do you mean to say that anyone holding these three papers, and without the seven others, could construct a Bruce-Partington submarine?"
"I reported to that effect to the Admiralty. But to-day I have been over the drawings again, and I am not so sure of it. The double valves with the automatic self-adjusting slots are drawn in one of the papers which have been returned. Until the foreigners had invented that for themselves they could not make the boat. Of course they might soon get over the difficulty."
"But the three missing drawings are the most important?"
"Undoubtedly."
"I think, with your permission, I will now take a stroll round the premises. I do not recall any other question which I desired to ask."
He examined the lock of the safe, the door of the room, and finally the iron shutters of the window. It was only when we were on the lawn outside that his interest was strongly excited. There was a laurel bush outside the window, and several of the branches bore signs of having been twisted or snapped. He examined them carefully with his lens, and then some dim and vague marks upon the earth beneath. Finally he asked the chief clerk to close the iron shutters, and he pointed out to me that they hardly met in the centre, and that it would be possible for anyone outside to see what was going on within the room.