Abstract: An improved method or process for the continuous manufacture of mats to be processed into composition boards is disclosed, the board-forming process being a dry process involving hot-pressing of the mats and curing of the binder. The mats comprise a fibrous or particulate material or materials, and a binder, the mats being dry. The mat material is laid onto a conveyor by formers or felters. In order to ensure that the resulting composition board has a uniform strength distribution and uniform specific gravity or density, a beta ray density measuring device measures successively the weight of air-borne material passing through a conduit or conduits to the forming apparatus, and the measured value is used to regulate the speed of the conveyor.