Page:Elementary Text-book of Physics (Anthony, 1897).djvu/177

§ 136] over its ianer surface. When the rod is made to vibrate longitudinally, the air-column in the tube, if of the proper length, is made to vibrate in unison with it. This agitates the powder and causes it to indicate the positions of the nodes in the vibrating air-column. The ratio of the velocity of sound in the solid to that in air is thus the ratio of the length of the rod to the distance between the nodes in the air-column.