Some embodiments relate to an apparatus comprising an elongate flexible tube sized to be received within a tract and having a proximal end and a distal end; a drive mechanism coupled to the proximal end of the tube; and a liquid column extending from the proximal end to the distal end; wherein the drive mechanism is configured to cause movement of the liquid column within the tube to impart forward momentum to the tube and thereby promote advancement of at least the distal end of the tube within the tract when at least the distal end is received within a part of the tract.