This I accomplish by the device illustrated in the accompanying drawings, in which- Figure 1 is an outside view of one section and a portion of an adjoining section of my shield as laid, inclosing the main and showing the escape-pipe; Fig. 2, across-section of Fig. 1 on line 00 m,- Fig. 3, a view of the main,with the outer half of the shield removed,on line y y, Fig. 2, and showing the packing .de