A sheet of paper being stretched tightly across a flat frame, if fine iron filings be dusted uniformly over it, and a fine pointed sewing needle, magnetised to saturation, held vertically, be moved below it, the point being kept at a short distance from the paper, the motion of the needle-point will be indicated by a fine line of filings which, attracted by the magnet, separate themselves from the homogeneous dust around. The fineness of this line will depend on the sharpness of the needle point, and the smallness of the particles of iron; the distance of the point from the paper will depend on the strength of the magnet, and the smoothness of the surface on which the particles lie. In some experiments already made, lines have been obtained as fine as could be produced by the sharpest pointed pencil, and this with a portion only of a small sewing needle. It will require a more extensive series of experiments to determine the best size for the magnet, and the best kind of paper for the filings. The particles of iron having a certain weight, are not easily shaken from their position if moderate care be taken.