Abstract: Sub-lance installation for carrying out measurements and/or taking samples in a metallurgical furnace, has an elongate tubular lance which is rotatable around its longitudinal axis and is attached at its upper end with its longitudinal axis vertical to a carriage by means of which the lance is moved vertically, the installation also including means for rotating the lance about its longitudinal axis. In order that the rotation of the sub-lance can be achieved by a robust apparatus, which does not itself require power and can operate automatically, said means for rotating the lance comprises a helical guide system which imparts rotation to the lance. The helical guide system comprises mutually cooperating helical projections and recesses mounted respectively on the lance and a guide for the lance.