(defun pjb-animate (speed)
  (interactive "nSpeed: ")
  (let ((delay (/ 1.0  speed))
        (done  nil))
    (widen)
    (goto-char (point-min))
    (message "Animating...")
    (while (not done)
      (widen)
      (if (search-forward "\f" nil 'at-limit)
          nil
        (goto-char (point-max))
        (setq done t))
      (narrow-to-page)
      (sit-for delay)
      (force-mode-line-update t))
    (message "Done.")))