lines by so doing the material for example snow as collected along the left edge of the track by the left half blade glides over the oblique surface of the latter half blade to the centre and is pushed forward by the right half blade if the latter is in the position shown in solid lines or the snow is discharged towards the right edge of the track if the right half blade is positioned in the position shown in dotted lines if both the half blade 17 and 18 are positioned obliquely forward as shown in fig5 it is possible to feed foward a larger mass of material along a longer path the orientation of the extensions 26 27 of the two half blades 17 18 for example as shown in fig6 makes it possible to match in a closer way and irregular ground profile the incline relative to the vertical of the oscillation axles 15 16 of the two half blades 17 18 serves also to improve the adaptability to the outline of the soil if the two half blades 17 18 are shifted rearwards see fig7 due to the incline of their oscillation axes the half blades adapt themselves to a convex track outline whereas with a forward displacement fig8 one obtains the adaption to a concave track outline the orientation of the two extensions 26 27 of the half blades 17 18 can also be automatically synchronized with the lifting and the depression of the intermediate frame 12 which carries the two half blades in the sense of properly lowering the extensions when the intermediate frame is lifted or vice versa so as to obtain that in the neutral position the two extensions and.