would have done.  It no longer matters.  He has to run to earth now before he is seen.  That is all his business.  And so the state of the car is explained.  It was a bold step to bring that car back—­yes, a bold and desperate step.  But a clever one.  For, if it had succeeded, we should have known nothing of their movements—­oh, but nothing—­ nothing.  Ah!  I tell you this is no ordinary blundering affair.  They are clever people who devised this crime—­clever, and of an audacity which is surprising.”

Then Hanaud lit another cigarette.

Mr. Ricardo, on the other hand, could hardly continue to smoke for excitement.

“I cannot understand your calmness,” he exclaimed.

“No?” said Hanaud.  “Yet it is so obvious.  You are the amateur, I am the professional—­that is all.”

He looked at his watch and rose to his feet.

“I must go” he said and as he turned towards the door a cry sprang from Mr. Ricardo’s lips “It is true.  I am the amateur.  Yet I have knowledge, Monsieur Hanaud which the professional would do well to obtain.”

Hanaud turned a guarded face towards Ricardo.  There was no longer any raillery in his manner.  He spoke slowly, coldly.

“Let me have it then!”

“I have driven in my motor-car from Geneva to Aix,” Ricardo cried excitedly.  “A bridge crosses a ravine high up amongst the mountains.  At the bridge there is a Custom House.  There—­at the Pont de la Caille—­your car is stopped.  It is searched.  You must sign your name in a book.  And there is no way round.  You would find sure and certain proof whether or no Madame Dauvray’s car travelled last night to Geneva.  Not so many travellers pass along that road at night.  You would find certain proof too of how many people were in the car.  For they search carefully at the Pont de la Caille.”

A dark flush overspread Hanaud’s face.  Ricardo was in the seventh Heaven.  He had at last contributed something to the history of this crime.  He had repaired an omission.  He had supplied knowledge to the omniscient.  Wethermill looked up drearily like one who has lost heart.

“Yes, you must not neglect that clue,” he said.

Hanaud replied testily: 

“It is not a clue.  M. Ricardo tells that he travelled from Geneva into France and that his car was searched.  Well, we know already that the officers are particular at the Custom Houses of France.  But travelling from France into Switzerland is a very different affair.  In Switzerland, hardly a glance, hardly a word.”  That was true.  M. Ricardo crestfallen recognized the truth.  But his spirits rose again at once.  “But the car came back from Geneva into France!” he cried.

“Yes, but when the car came back, the man was alone in it,” Hanaud answered.  “I have more important things to attend to.  For instance I must know whether by any chance they have caught our man at Marseilles.”  He laid his hand on Wethermill’s shoulder.  “And you, my friend, I should counsel you to get some sleep.  We may need all our strength tomorrow.  I hope so.”  He was speaking very bravely.  “Yes, I hope so.”