The door of the latter apartment was still open, and at the sight of the big, brilliantly lit interior he pulled up with an exclamation of surprise.
"Hullo!" he remarked. "I'd no idea it was such a size. The old man must have been pretty well off if he could afford to run up places like this."
He glanced round the room as though in search of the additional exit, and, without waiting for his question, Colin pointed toward a high screen which jutted out at right angles from the wall.
"It's behind there," he explained. "I never thought of looking to see if it was open. The Professor told me that he only used it in summer time."
"He used it to-night," was the detective's reply. "If he hadn't he would probably be alive now."
As he spoke he descended the steps, and, with Colin in close attendance, strode confidently toward the spot. They came to a halt in front of a small oak door, flush with the wall, and, catching hold of the handle, Marsden gave it a sharp turn. The next moment a gust of cold wind was blowing in their faces, and they were staring across the lawn in the direction of the study windows, from which a flood of yellow light streamed out into the darkness of the garden.
It was the Inspector who first broke the silence. "That's clear enough as far as it goes," he observed. "The question is, Why did he open the door at one o'clock in the morning?"
A possible explanation suddenly occurred to Colin.
"I shouldn't wonder if he wanted to let in a little fresh air. He'd been making an experiment, and there was a horrible smell in the room when I spoke to him at the doorway."
"You've got it," was Marsden's laconic answer. He pulled out an electric torch, a duplicate of the sergeant's, and allowed the light to play backward and forward over the patch of gravel outside. "I don't suppose there will be any footprints," he continued. "It's been freezing too infernally hard for that, and, in any case, we shall only do more harm than good by trampling all over the place in the dark." He switched off the torch, and closing and locking the door, put away the key in his pocket.