exerted upon the guiding bow 1 are the drift force f d and the lift force f l it is generally known in the art of aerodynamics that the magnitude of these two forces f d and f l are dependent upon the cross sectional profile of the guiding bow 1 and upon the direction of the profile with respect to the direction of movement v this direction may be characterized by the angle u03b1 the cross sectional profile of the bow and the angle u03b1 should be chosen so as to minimize the drift force f d the cross sectional profile of the bow and the angle u03b1 should also be chosen such that the lift force f l has a sense of direction different from the sense of direction of centrifugal force f c since in contradistinction with airplanes a lift of the bow is not desired here in case this is not possible the cross sectional profile of the bow and the angle u03b1 should be so chosen that the lift force f l is as small as possible fig2 illustrates another embodiment of the present invention the core 5 is surrounded by a sheath 7 which gives the streamlined profile to the guiding bow 1 the sheath 7 is such that it may.