He disappeared in the darkness, leaving the others rather puzzled as to the meaning of these manœuvres. At last Wendover thought he saw the point.
“I see what he's getting at.”
But just as he was about to explain the matter to the inspector, they saw the flash of Sir Clinton's lamp and Armadale set off at a lumbering trot across the sands with Wendover hurrying after him.
“It's simple enough,” Sir Clinton explained when all three had gathered at Neptune's Seat. “You remember that Billingford's track was broken where the cairns are—no footprints visible for several yards. That was the place where he crossed the runnel last night. All we need to do is to note when the runnel is the same breadth at that point to-night—which we've just done—and then make a correction of about forty minutes for the tide being later this evening. It's not exact, of course; but it's near enough, perhaps.”
“I thought you were after something of the sort,” Wendover interjected. “Once I got as far as the runnel I tumbled to the idea.”
“Well, let's take the results,” Sir Clinton went on. “The runnel was in the right state to-night at five past twelve. Make the forty-one minute deduction—since the tide's forty-one minutes later to-night than it was last night—and you get 11.25 p.m. as the time when Billingford crossed that rivulet last night. Now the inspector took over seven minutes to run from the cairns to Neptune's Seat, for I timed him. Therefore, even if Billingford had run the whole distance, he couldn't have reached Neptune's Seat before 11.32 at the earliest. As a matter of fact, his track showed that he walked most of the way, which makes the possible time of his arrival a bit later than 11.32 p.m.”
“So he couldn't have fired a shot at 11.19 when Staveley's watch stopped?” Wendover inferred.
“Obviously he couldn't. There's more in it than that; though I needn't worry you with that at present, perhaps. But this bit of evidence eliminates another possibility I'd had my eye on. Billingford might have walked into the runnel and then waded down the rivulet into the sea, leaving no traces. Then he might have come along the beach, wading in the waves, and shot Staveley from the water. After that, if he'd returned the way he came, he could have emerged from the runnel at the same point, only on this bank of the channel, and left his single track up to Neptune's Seat, just as we found it. But that won't fit in with the shot fired at 11.19 p.m., obviously. If he'd done this, then his last footmark on the far side would have been made when the runnel was full, and his first footmark on this side would have been made later, when the runnel had shrunk a bit; and the two wouldn't have fitted the banks neatly as we found to-night that they did.”
“I see all that, clear enough, sir,” said Armadale briskly. “That means the circle's narrowed a bit further. If Billingford didn't fire that shot, then you're left with only three other people on the list: the two Fleetwoods and the woman with the 3½ shoe. If she can be eliminated like Billingford, then the case against the Fleetwoods is conclusive.”
“Don't be in too much of a hurry, inspector. How are you getting along with the shoe question?” Sir Clinton inquired a trifle maliciously.