Elliott Bros, London Abney level no 1796, registered no 5205 Capt. Abney designed  registered in 1884  this  is an early example , probably dating around 1890 . Top bar is a level tube which is rotated by the milled hand wheel. In use the object of which the height is to be determined is sighted through the instrument and the bubble tube is rotated until the bubble, viewed in a mirror in the sighting tube, is brought to the same position as the top of the sighted object.