A device for moving a jet member having a nozzle, said device comprising • a carriage provided with a base portion to which said jet member is connected, • a guide member along which said carriage is movable in a substantially rectilinear first path for moving the nozzle of the jet member over a layer to be treated by the jet, the connection of said jet member to said base portion providing a movability of the jet member with respect to the base portion and by that said carriage comprising a first movability of the jet member for moving an impact point of said jet in a second path perpendicularly to said first path, • a driving arrangement (9, 10, 21)configured to move said carriage along said first path and said jet member with respect to said base portion of said carriage, and • a control unit configured to control said driving arrangement for controlling the movement of said impact point of said jet over a said layer, characterized in that the control unit is configured to control said driving arrangement to combine a movement of said carriage along said guide member and a movement of said jet member with respect to said base portion of said carriage so as to make said impact point of said jet on a said layer to travel with a constant speed over said layer, that the control unit is configured to control the driving arrangement to obtain a speed of the movement of said jet member along said second path having a maximum in a mid-region along this path and decreasing towards end positions of said second path for increasing after returning from a said end position and to compensate a change of speed of said impact point along said second path caused thereby by means of a corresponding opposite change of speed of the impact point along said first path, that said movability of the jet member with respect to said base portion of the carriage comprises a second movability of the jet member along said first path, and that the control unit is configured to control said driving arrangement to move the jet member with respect to said base portion along said first path with a frequency being twice the frequency of a movement of the jet member with respect to the base portion along said second path and the jet member to be in a mid-region of the movement according to said second movability with a maximum speed of this movement and to move in the direction of movement of said carriage along said first path at said end positions of the movement of the jet member according to said first movability. • a carriage provided with a base portion to which said jet member is connected, • a guide member along which said carriage is movable in a substantially rectilinear first path for moving the nozzle of the jet member over a layer to be treated by the jet, the connection of said jet member to said base portion providing a movability of the jet member with respect to the base portion and by that said carriage comprising a first movability of the jet member for moving an impact point of said jet in a second path perpendicularly to said first path, • a driving arrangement (9, 10, 21)configured to move said carriage along said first path and said jet member with respect to said base portion of said carriage, and • a control unit configured to control said driving arrangement for controlling the movement of said impact point of said jet over a said layer,