"Every one of the cabs is fitted with a telephonic arrangement communicating with the roof. The driver has only to button the wire of the transmitter up inside his coat so that the transmitter itself lies near to his ear, and he can hear even a whisper inside the cab. "The man who is driving us, for instance, has a sort of retainer from the Russian Embassy to be on hand at certain hours on certain nights in the week. Our cabs are all better horsed, better appointed, and better driven than any others in London, and, consequently, they are favourites, especially among the young attachés, and are nearly always employed by them on their secret missions or love affairs, which, by the way, are very often the same thing. Our own Jehu has a job on to-night, from which we expect some results that will mystify the enemy not a little. We got our first suspicions of Ainsworth from a few incautious words that he spoke in one of our cabs." "It's a splendid system, I should think, for discovering the movements of your enemies," said Arnold, not without an uncomfortable reflection on the fact that he was himself now completely in the power of this terrible organisation, which had keen eyes and ready hands in every capital of the civilised world. "But how do you guard against treachery? It is well known that all the Governments of Europe are spending money like water to unearth this mystery of the Terror. Surely all your men cannot be incorruptible." "Practically they are so. The very mystery which enshrouds all our actions makes them so. We have had a few traitors, of course; but as none of them has ever survived his treachery by twenty-four hours, a bribe has lost its attraction for the rest." In such conversation as this the time was passed, while the cab crossed the river and made its way rapidly and easily along Kennington Road and Clapham Road to Clapham Common. At length it turned into the drive of one of those solid abodes of pretentious respectability which front the Common, and pulled up before a big stucco portico. "Here we are!" exclaimed Colston, as the doors of the cab automatically opened. He got out first, and Arnold handed the case to him, and then followed him. Without a word the driver turned his horse into the road again and drove off towards town, and as they ascended the steps the front door opened, and they went in, Colston saying as they did so-- "Is Mr. Smith at home?" "Yes, sir; you are expected, I believe. Will you step into the drawing-room?" replied the clean-shaven and immaculately respectable man-servant, in evening dress, who had opened the door for them. They were shown into a handsomely furnished room lit with electric light. As soon as the footman had closed the door behind him, Colston said--