Page:Popular Science Monthly Volume 49.djvu/326

310 support of the vortex theory. Fig. 4 shows the marks left by a stroke of lightning on the wall of a house at Mort, illustrating its course from the iron water pipe to the iron hinges of the window, and from thence to the iron sewer grating. For comparison he sends two other photographs (Figs. 5 and G). These are obtained by the discharge of an immense Ruhmkorff induction coil containing one hundred thousand metres of insulated copper wire. Two small triangles of tin foil are glued opposite to one another on a photographic plate, and when dry the whole is



covered with a light brownish coating of lampblack. After the discharge a white track is found, as shown in the photographs. Both these pictures show evidence of the vortex motion of the