It was very curious. He'd been walking close to the white-tiled wall, and I saw him suddenly stop; but he didn't turn. He didn't even turn when I pulled up, close behind him; he put out one hand to the wall, as if to steady himself. But, the moment I touched his shoulder, he just dropped--just dropped, half on his knees against the white tiling. The face he turned round and up to me was transfixed with fright. There were half a hundred people about--a train was just in--and it isn't a difficult matter in London to get a crowd for much less than a man crouching terrified against a wall, looking over his shoulder as Rooum looked, at another man almost as terrified. I felt somebody's hand on my own arm. Evidently somebody thought I'd knocked Rooum down. The terror went slowly from his face. He stumbled to his feet. I shook myself free of the man who held me and stepped up to Rooum. "What the devil's all this about?" I demanded, roughly enough. "It's all right ... it's all right,..." he stammered. "Heavens, man, you shouldn't play tricks like that!" "No ... no ... but for the love of God don't do it again!..." "We'll not explain here," I said, still in a good deal of a huff; and the small crowd melted away--disappointed, I dare say, that it wasn't a fight. "Now," I said, when we were outside in the crowded street, "you might let me know what all this is about, and what it is that for the love of God I'm not to do again." He was half apologetic, but at the same time half blustering, as if I had committed some sort of an outrage. "A senseless thing like that!" he mumbled to himself. "But there: you didn't know.... You _don't_ know, do you?... I tell you, d'you hear, _you're not to run at all when I'm about_! You're a nice fellow and all that, and get your quantities somewhere near right, if you do go a long way round to do it--but I'll not answer for myself if you run, d'you hear?... Putting your hand on a man's shoulder like that, just when ..." "Certainly I might have spoken," I agreed, a little stiffly. "Of course, you ought to have spoken! Just you see you don't do it again. It's monstrous!" I put a curt question. "Are you sure you're quite right in your head, Rooum?" "Ah," he cried, "don't you think I just fancy it, my lad! Nothing so easy! I thought you guessed that other time, on the new road ... it's as plain as a pikestaff... no, no, no! _I_ shall be telling _you_ something about molecules one of these days!" We walked for a time in silence. Suddenly he asked: "What are you doing now?" "I myself, do you mean? Oh, the firm. A railway job, past Pinner. But we've a big contract coming on in the West End soon they might want you for. They call it 'alterations,' but it's one of these big shop-rebuildings." "I'll come along." "Oh, it isn't for a month or two yet." "I don't mean that. I mean I'll come along to Pinner with you now, to-night, or whenever you go." "Oh!" I said.