A wadded mattress should be prepared and placed behind the canvas or paper, and between it and a sheet of cardboard fixed on the frame by screws. Mineral cotton in spun glass or a stuffing of asbestos may be used, as they are both indestructible by fire or dampness. The cardboard or shelf which is to give resistence behind should be carefully rendered waterproof. Instead of the cotton, cork or any similar body might be used, provided the body were impervious and very light, and had no tendency to sink to the bottom of the frame, and that its pressure remained equally distributed over all the surface of the paper or the canvas.,