essence of the invention tube 100 is pictured to be hollow but it is obvious to use a solid member or one having more complex internal structure tube 100 has an exterior surface 102 which defines the circumference of the tube 100 exterior surface 102 is interrupted by peripheral wall 104 which cuts into the tube 100 peripheral wall 104 and bottom 106 together define the outline and cross section of recess 108 the cross section of tube 100 with recess 108 in a plane perpendicular to the axis of tube 100 may be described as the intersection of two partial chords of tube 100 in this case neither bottom 106 nor peripheral wall 104 cut completely across tube 100 from one point on its circumference to another as the partial chord defined by the peripheral wall 104 is radial within the cross sectional plane the recess 108 is only a fraction of the section which would be defined by two radii meeting the circumference of the tube 100 at the same locations as the partial chords defined by bottom 106 and peripheral wall 108 this section defined by recess 108 will be referenced herein as a partial chordal section note too that bottom 106 necessarily has the geometric property of being parallel to some tangent of the circumference of tube 100 the points at which a tangent is parallel to a chord on a circular surface are those points which are directly perpendicular to the.