In the accompanying drawing, where three forms of the bristle section and two forms for anchoring the rotatable member are shown, Fig. 1 is a plan view of the preferred form of our invention, Fig. 2 is a side elevation of the brush shown in Fig. 1 with the bristle section concave and an intermediate step of changing the formn of the bristle section is indicated in dotted lines, Fig. 3 is a view si