How did Screwtape distract the man in the British museum from listening to the enemy?

From the text:

I once had a patient, a sound atheist, who used to read in the British Museum. One day, as he sat reading, I saw a train of thought in his mind beginning to go the wrong way. The Enemy, of course, was at his elbow in a moment. Before I knew where I was I saw my twenty years' work beginning to totter. If I had lost my head and begun to attempt a defence by argument I should have been undone. But I was not such a fool. I struck instantly at the part of the man which I had best under my control and suggested that it was just about time he had some lunch.