Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: FIGURE 1 is an axial section of a rotary seal embodying the present invention; FIG. 2 is a developed section taken in a plane indicated by the line 2-2 in FIG. 1; FIG. 3 is an axial section of a second type of seal also embodying the inven