A surface, e.g., a concrete surface, is worked by guiding a lance (23) which comprises a water jet mouthpiece (27) coupled to the high-pressure water over the surface in a working path and at the same oscillating the lance transversely to the working path. The oscillation is driven by an eccentric mechanism (30, 31) and has a stable eccentric position when the mechanism is rotated in the one direction and a stable eccentric position when it is rotated in the other direction. The two positions have a different eccentricity and impart different widths to the working. The working depth and the working width can be changed by changing the direction of rotation.

A device for moving a jet member (6) having a nozzle comprises a carriage (4) with a base portion (5) to which the jet member is connected. The carriage is movable in a substantially rectilinear first path along a guide member (3). The connection of the jet member to the base portion provides a movability of the jet member (6) with respect to the base portion (5) and by that said carriage comprising a movability of the jet member for moving an impact point of the jet in a second path perpendicularly to said first path. A control unit is controlling a driving arrangement to combine a movement of the carriage along the guide member (3) and a movement of the jet member with respect to the base portion of the carriage so as to make an impact point of the jet on a said layer to travel with a constant speed over the layer.