THE BRITISH MUSEUM IS FALLING DOWN by David Lodge. 176 pages. Holt, Rinehart & Winston. $3.95.
"How different it must be," muses Adam Appleby as his day begins, "the life of an ordinary, non-Catholic parent, free to decide—actually to decide, in calm confidence—whether or not to have a child." But the Roman Catholic Church, to which the Applebys adhere, and the rhythm method, to whose uncertain discipline they reluctantly submit, allow no such latitude. Their first child arrived nine months after the wedding, followed, at similar intervals, by two more. And now, dabbing queasily at the breakfast bacon, Adam's wife congeals...