Abstract: An apparatus for detecting a selected liquid contaminant includes a material holder and a reactive material held by the material holder. The reactive material absorbs a selected liquid contaminant with a measurable change reaction occurring as a result of such absorption. A signal generator is provided for generating a signal upon the measurable change reaction exceeding a preset threshold.

Abstract: An apparatus for displaying a series of stationary images to form an animated display when seen from a moving subway train is presented. A series of image display panels are located along a length of subway track. These panels are momentarily illuminated each time a passing train window passes a predetermined point relative to the panel by means of a sensor device attached to each panel. The sensor device detects the presence of a nearby train window by means of a light signal received from each such window. The light signal may be derived either from a source associated with each window or from a reflector associated with each window reflecting light from a source attached to each stationary panel.