“The data given are taken from the Journal of Experimental Medicine, reprints kindly furnished by Major George A. Stewart of the Rockefeller Institute. The diagrams are reproduced from the issue of Feb. 1, 1918, pp. 171 and 172, article by Dr. T. Tuffier and R. Desmarres, Auxiliary Hospital 75, Paris.... [ A graph, showing the progress of healing of a surface wound of the right leg of a 31-year-old patient. It shows that as time passes, the wounded area decreases. ] Progress of healing of a surface wound of the right leg, patient’s age 31 years. “WAVE MOTION. General.—In nature there are two types of recurrent motion, somewhat closely connected mathematically, in which repetition of motion occurs at regular intervals. “One type of this motion, in cycles as we may say, repeats the motion in one place, and is in a sense stationary. The tuning fork in motion moves through the same space again and again; a similar movement is the motion of a vibrating string. Of this stationary type may be mentioned the heartbeats, the pulse, the respiration, the tides, and the rotation of a wheel about its axis. “The second type of recurrent motion transmits or carries the vibratory impulse over an extent of space as well as time. The waves of the sea are of this character. Sound waves, electrical vibrations or waves, and radiant energy vibrations are transmitted by a process similar to that by which the waves of the sea are carried. “Both of these types of motion are representable mathematically by equations involving a sequence of trigonometric functions. To the fundamental and basic function involved, _y_ = sin _x_, we will direct our attention in the next section and to simple applications in other sections of this chapter.... “Sound Waves.—If a tuning fork for note lower C is set to vibrating, the free bar makes 129 complete, back-and-forth, vibrations in one second. By attaching a fine point to the end of the bar and moving under this bar at a uniform rate, as it vibrates, a smoke-blackened paper, a sinusoidal curve is traced on the paper. Our curve is traced by a bar vibrating 50 times in 1 second. [ A graph, showing a wavy line. “Tuning fork vibrations recorded on smoked paper.” ] Tuning fork vibrations recorded on smoked paper.