smoke-signal   scheme

;;
;; Smoke signal injects smoke into the system
;; at semi-regular intervals (* 40000 (random 4 9))
;;
(define smoke-signal
  (lambda (time)
    ;; increase 300.0 to kick more smoke into the system
    (add-density 2 5 11 (+ 1.0 (* (random) 50.0)))
    ;; decrease (random 4 9) to inject smoke more often
    (callback (+ time 4410) 'smoke-signal (+ time (* 40000 (random 4 9))))))