"Wait until someone comes out of the shop and is driven off in it; then follow. Do not lose sight of the cab for a moment. When it draws up, and wherever it draws up, drive right past it. Don't attract attention by stopping. You understand?"
"Quite, sir," said the man, smiling slightly. And Cairn entered the cab.
The cabman drew up at a point some little distance beyond, from whence he could watch. Two minutes later Ferrara came out and was driven off. The pursuit commenced.
His cab, ahead, proceeded to Westminster Bridge, across to the south side of the river, and by way of that commercial thoroughfare at the back of St. Thomas' Hospital, emerged at Vauxhall. Thence the pursuit led to Stockwell, Herne Hill, and yet onward towards Dulwich.
It suddenly occurred to Robert Cairn that Ferrara was making in the direction of Mr. Saunderson's house at Dulwich Common; the house in which Myra had had her mysterious illness, in which she had remained until it had become evident that her safety depended upon her never being left alone for one moment.
"What can be his object?" muttered Cairn.
He wondered if Ferrara, for some inscrutable reason, was about to call upon Mr. Saunderson. But when the cab ahead, having passed the park, continued on past the lane in which the house was situated, he began to search for some other solution to the problem of Ferrara's destination.
Suddenly he saw that the cab ahead had stopped. The driver of his own cab without slackening speed, pursued his way. Cairn crouched down upon the floor, fearful of being observed. No house was visible to right nor left, merely open fields; and he knew that it would be impossible for him to delay in such a spot without attracting attention.
Ferrara's cab passed:
"Keep on till I tell you to stop!" cried Cairn.