Martin Hackness dressed with less than his usual care that evening. He even forgot that Miss Cynthia Grimfern had a strong prejudice in favour of black evening ties, and, usually, he paid a great deal of deference to her opinions. But he was thinking of other matters now.

There was no sign of anything abnormal as Hackness drove along in the direction of Clarence Terrace. The night was more than typically yellow for the time of year, but there was no kind of trouble with the traffic though down the river the fairway lay under a dense bank of cloud.

Hackness sniffed the air eagerly. He detected or thought he detected a certain acrid suggestion in the atmosphere. As the cab approached Trafalgar Square Hackness could hear shouts and voices raised high in protestation. Suddenly his cab seemed to be plunged into a wall of darkness.

It was so swift and unexpected that it came with the force of a blow. The horse appeared to have trotted into a bank of dense blackness. The wall had shut down so swiftly, blotting out a section of London, that Hackness could only gaze at it with mouth wide open.

Hackness hopped out of his cab hurriedly. So sheer and stark was the black wall that the horse was out of sight. Mechanically the driver reined back. The horse came back to the cab with the dazzling swiftness of a conjuring trick. A thin stream of breeze wandered from the direction of Whitehall. It was this air finding its way up the funnel formed by the sheet that cut off the fog to a razor edge.

"Been teetotal for eighteen years," the cabman muttered, "so that\'s all right. And what do you please to make of it, sir?"

Hackness muttered something incoherent. As he stood there, the black wall lifted like a stage curtain, and he found himself under the lee of an omnibus. In a dazed kind of way he patted the cabhorse on the flank. He looked at his hand. It was greasy and oily and grimy as if he had been in the engine-room of a big liner.

"Get on as fast as you can," he cried. "It was fog, just a little present from the burning petroleum. Anyway, it\'s gone now."

True, the black curtain had lifted, but the atmosphere reeked with the odour of burning oil. The lamps and shop windows were splashed and mottled with something that might have passed for black snow. Traffic had been brought to a standstill for the moment, eager knots of pedestrians were discussing the situation with alarm and agitation, a man in evening dress was busily engaged in a vain attempt to remove sundry black patches from his shirt front.

Sir Edgar Grimfern was glad to see his young friend. Had Grimfern been comparatively poor, and less addicted to big game shooting, he would doubtless have proved a great scientific light. Anything with a dash of adventure fascinated him. He was enthusiastic on flying machines and aeroplanes generally. There were big work-shops at the back of 119, Clarence Terrace, where Hackness put in a good deal of his spare time. Those two were going to startle the world presently.

Hackness shook hands thoughtfully with Cynthia Grimfern. There was a slight frown on her pretty intellectual face as she noted his tie.

"There\'s a large smut on it," she remarked, "and it serves you right."

Hackness explained. He had a flattering audience. He told of the strange happening in Trafalgar Square and the majestic scene on the river. He gave a graphic account of the theory that he had built upon it. There was an animated discussion all through dinner.

"The moral of which is that we are going to be plunged into Cimmerian darkness," Cynthia said, "that is, if the fog comes down. If you think you are going to frighten me............