I can't say how many minutes.

Perhaps a quarter of an hour.

But not suspecting anything was going to happen, I didn't look at my watch." Jennings looked thoughtfully at the carpet.

"I wonder if the assassin escaped that way," he murmured.
"Which way ?" "Over the wall and through the park.

You see, he could not have gone up the lane or through the railway path without stumbling against that policeman.