With diaries, as with mustard, one always feels that it cannot be what people use that makes the manufacturers' profits; it must be what they waste. Keeping a diary has become a New Year's joke, and for all that most people are concerned, the diary might have nothing but blank pages after about January 12. Mr. Ponsonby, however, will say a good word for the diary habit. He himself, in the intervals of a diplomatic and political career (which led him to the Under-Secretaryship for Foreign Affairs in the Labour Government three years ago) has studied English diaries, published anthologies of them, and developed an enthusiasm for them, that he will try to communicate to his listeners tonight.