A method of determining the angular inclination of a borehole in which a glass, testtubelike bottle partly filled with a dilute solution of hydrofluoric acid is inserted in a watertight metal case. When the assemblage is lowered into a borehole and left for 20 to 30 min, the acid etches the bottle at a level plane from which the inclination of the borehole can be measured. Compare: Kiruna method.