ONE half of the world, so the platitude runs, has no idea how the other half lives; it is equally true that those whose work occupies normal daytime hours have very little comprehension of the activities that go on while they sleep. Especially is this so in the great cities. In this series of talks men and women of practical experience will describe the life that is lived (in this case in London), while most of us are sleeping. This time the talk will be given by a Covent Garden porter, who will describe the great market during the night and early morning hours.