“But you might have stuck to Mordecai Smith, and so been led to their hiding-place,” said I.

“In that case I should have wasted my day. I think that it is a hundred to one against Smith knowing where they live. As long as he has liquor and good pay, why should he ask questions? They send him messages what to do. No, I thought over every possible course, and this is the best.”

While this conversation had been proceeding, we had been shooting the long series of bridges which span the Thames. As we passed the City the last rays of the sun were gilding the cross upon the summit of St. Paul’s. It was twilight before we reached the Tower.

“That is Jacobson’s Yard,” said Holmes, pointing to a bristle of masts and rigging on the Surrey side. “Cruise gently up and down here under cover of this string of lighters.” He took a pair of night-glasses from his pocket and gazed some time at the shore. “I see my sentry at his post,” he remarked, “but no sign of a handkerchief.”

“Suppose we go downstream a short way and lie in wait for them,” said Jones eagerly.

We were all eager by this time, even the policemen and stokers, who had a very vague idea of what was going forward.

“We have no right to take anything for granted,” Holmes answered. “It is certainly ten to one that they go downstream, but we cannot be certain. From this point we can see the entrance of the yard, and they can hardly see us. It will be a clear night and plenty of light. We must stay where we are. See how the folk swarm over yonder in the gaslight.”

“They are coming from work in the yard.”

“Dirty-looking rascals, but I suppose every one has some little immortal spark concealed about him. You would not think it, to look at them. There is no a priori probability about it. A strange enigma is man!”

“Someone calls him a soul concealed in an animal,” I suggested.

“Winwood Reade is good upon the subject,” said Holmes. “He remarks that, while the individual man is an insoluble puzzle, in the aggregate he becomes a mathematical certainty. You can, for example, never foretell what any one man will do, but you can say with precision what an average number will be up to. Individuals vary, but percentages remain constant. So says the statistician. But do I see a handkerchief? Surely there is a white flutter over yonder.”

“Yes, it is your boy,” I cried. “I can see him plainly.”

“And there is the Aurora,” exclaimed Holmes, “and going like the devil! Full speed ahead, engineer. Make after that launch with the yellow light. By heaven, I shall never forgive myself if she proves to have the heels of us!”

She had slipped unseen through the yard-entrance and passed between two or three small craft, so that she had fairly got her speed up before we saw her. Now she was flying down the stream, near in to the shore, going at a tremendous rate. Jones looked gravely at her and shook his head.

“She is very fast,” he said. “I doubt if we shall catch her.”

“We must catch her!” cried Holmes between his teeth. “Heap it on, stokers! Make her do all she can! If we burn the boat we must have them!”