levelling device at least one lateral jack is provided whose cylinder 17 is articulated at 9c on frame 9 while the piston rod 18 is coupled to a lug 10a on arm 10 the mean position of this lug 10a is perpendicular to the axis of jack 17 18 so that actuation of the latter is translated by a rotation of the arms 10 and of the shaft 5 causing the passage from one to the other of the arrangements shown in fig5 and 6 here too a stop 13 39 is provided to limit the amplitude of the angular displacement of the connecting rods 13 operation of the device is similar to that set forth with reference to fig2 to 4 effectively the end referenced 9a of the slots 9b which faces shaft 4 replaces the bearing surfaces 9a of fig2 to 4 under these conditions in the position according to fig5 the connecting rods 13 apply to the pivot pin 5 an elastic pre load which opposes any untimely displacement of the pin while in the levelled position of fig6 the actuation effected by the jack or jacks 17 18 has brought the assembly of the levers 1 in abutment against the fixed stop 8 by transverse sliding of the ends 5a of this pin 5 in the slots 9b in this case too the locked position of equilibrium according to fig5 i e after the point of maximum extension of the connecting rods 13 has been exceeded is rendered stable by the presence of.