The late war gave prominence to certain acoustic phenomena which, though hardly mysterious, were novel to the world at large. One of those was the double report (triple in the case of an exploding shell) heard near the line of fire of large guns. This effect is due to the fact that modern projectiles travel much faster than sound. The moving projectile sets up its own waves in the air, like those at the bow of a steamer, which may reach the ear of the observer and produce the sensation of a sharp sound before he hears the sound coming from the mouth of the gun. Another phenomenon frequently observed when heavy firing was in progress was the appearance in the sky of rapidly moving parallel arcs of light and shade. These were generally seen against clouds, but sometimes they swept across blue sky. They probably occurred only in calm weather. These arcs were the result of the successive condensations and rarefactions of the air constituting waves of sound--visible sound waves. Their visibility was due to contrasts in the refraction of light passing through air of different densities; the same sort of refraction contrasts that cause the tremulous appearance of the air over a hot stove, for example. The same “flashing arcs” of light have been described by Prof. F. A. Perret as attending explosive volcanic outbursts at the craters of Vesuvius and Ætna. The humming of telegraph wires has been the subject of a certain amount of discussion in meteorological circles, but without altogether satisfactory results. This sound is not, of course, caused or affected by the electric currents passing along the wire, and it is almost certainly due solely to the wind, though the suggestion has been made that it might be caused by the microseisms, or small and rapid earthquake tremors, that are so commonly registered by seismographs while imperceptible to the human senses. The humming is best heard when one’s ear is placed against a telegraph pole. Several persons have made systematic observations of these sounds from day to day, and it has often been alleged that they vary with the temperature, the movements of storms, etc., and even constitute a safe basis for weather predictions. They are sometimes heard when the air appears to be perfectly calm, but in such cases there might be some movement of the air at the level of the wires, though there was none at the lower level of the observer. From what is known about “æolian tones” (such as those of the æolian harp), it would seem that the humming requires a wind more or less at right angles to the wire, and that the pitch of the sound depends upon the force of the wind and the diameter (but not the length or tension) of the wire. For a given wire, the stronger the wind the higher the pitch of its sound.