An instrument comprising a cable or tube, at a distal end of which a propulsion device is provided for moving the cable or tube in a hollow space, the propulsion device being shaped like a donut lying in a plane at right angles to the longitudinal direction of the cable or tube, wherein the donut-shaped propulsion device is, at least in part, externally delimited by at least one wire gauze that is rotatable about a closed axis of the donut body, which axis lies in the plane of the donut.