An instrument to assess the strength of a coal seam, its relative workability, and the influence of roof pressure. It consists of a steel rod of sectional area 1/4 in₂(1.6 cm₂) that is pushed into the coal, normal to the coal face, under the action of a light hydraulic ram. The ram is braced against lightweight props erected at the face. When in position, the penetrometer gives a graph of load against penetration at a particular point. Readings are taken at a number of points laterally and vertically along the face, and these can be correlated with the performance of plow-type machines. Thus, the probable performance of a machine in a seam can be estimated without the need for costly trials.