His cab was just on the point of turning on to the slope of Vauxhall Bridge.  And fifty yards behind, speeding along the Embankment, was a small French car.  The features of the driver he had no time to observe.  But, peering eagerly through the window, showed the dark face of the passenger.  The man’s nationality it was impossible to determine, but the keen, almost savage interest, betrayed by the glittering black eyes, it was equally impossible to mistake.

If the following car had turned on to the bridge, Harley, even yet, might have entertained a certain doubt.  But, mentally putting himself in the pursuer’s place, he imagined himself detected and knew at once exactly what he should do.  Since this hypothetical course was actually pursued by the other, Harley’s belief was confirmed.

Craning his neck, he saw the little French car turn abruptly and proceed in the direction of Victoria Station.  Instantly he acted.

Leaning out of the window he thrust a ten-shilling note into the cabman’s hand.  “Slow down, but don’t pull up,” he directed.  “I am going to jump out just as you pass that lorry ahead.  Ten yards further on stop.  Get down and crank your engine, and then proceed slowly over the bridge.  I shall not want you again.”

“Right-oh, sir,” said the man, grinning broadly.  As a result, immediately he was afforded the necessary cover, Harley jumped from the cab.  The man reached back and closed the door, proceeding on his leisurely way.  Excepting the driver of the lorry, no one witnessed this eccentric performance, and Harley, stepping on to the footpath, quietly joined the stream of pedestrians and strolled slowly along.

He presently passed the stationary cab without giving any sign of recognition to the dismounted driver.  Then, a minute later, the cab overtook him and was soon lost in the traffic ahead.  Even as it disappeared another cab went by rapidly.

Leaning forward in order to peer through the front window was the dark-faced man whom he had detected on the Embankment!

“Quite correct,” murmured Harley, dryly.  “Exactly what I should have done.”

The spy, knowing himself discovered, had abandoned his own car in favour of a passing taxicab, and in the latter had taken up the pursuit.

Paul Harley lighted a cigarette.  Oddly enough, he was aware of a feeling of great relief.  In the first place, his sixth sense had been triumphantly vindicated; and, in the second place, his hitherto shadowy enemies, with their seemingly supernatural methods, had been unmasked.  At least they were human, almost incredibly clever, but of no more than ordinary flesh and blood.

The contest had developed into open warfare.  Harley’s accurate knowledge of London had enabled him to locate No. 236 South Lambeth Road without recourse to a guide, and now, walking on past the big gas works and the railway station, he turned under the dark arches and pressed on to where a row of unprepossessing dwellings extended in uniform ugliness from a partly demolished building to a patch of waste ground.