The Brocken specter and the glory have occasionally been photographed.
CHAPTER XI
ATMOSPHERIC ACOUSTICS
Though air is but one of an unlimited number of elastic substances that transmit sound, it is the one through which sounds ordinarily reach our ears. Hence the acoustic properties of the atmosphere are of great interest to mankind.
Science deals with several kinds of “waves,” and those of the atmosphere that produce the sensation of sound are quite different from the waves of the sea. In quiet unconfined air sound travels in concentric spherical waves, consisting of successive condensations and rarefactions of the medium. Sound is not transmitted through a vacuum. A familiar laboratory experiment is to install an electric bell inside the receiver of an air pump and notice the dying away of the sound as the air is exhausted. The colossal eruptions that astronomers witness on the surface of the sun would probably be audible on earth if interplanetary space were filled with air. In the rarefied air of high mountains the intensity of sounds is much reduced. Thus we are told that on the top of Mont Blanc the report of a pistol sounds no louder than that of a firecracker at sea level.
The speed with which sound travels through air depends upon the temperature. At 32° F. (the freezing point) it is 1,087 feet per second, and at 68° F. it is 1,126 feet per second. The increase of speed with increase of air temperature is very close to 2 feet per degree Centigrade, or a little more than 1 foot per degree Fahrenheit. The sounds of violent explosions travel considerably faster than ordinary sounds near the place of explosion, but slow down to the normal speed at greater distances. This is true of heavy claps of thunder. The effect is not important enough, however, to invalidate the well-known rule that, if you count seconds between the flash and the detonation and divide the result by 5, you get approximately the distance of the source of sound in miles.
Since the speed of sound varies with the temperature of the air, differences in the latter cause deviations of the paths of sound waves similar to the deviations which rays of light undergo on account of differences in the density of the air. Sound is also reflected by obstacles, in the same manner as light. Moreover, whereas light travels too swiftly to be affected by the wind, this is not true of sound. The latter travels faster with the wind than against it, and sound waves are more or less broken up by the gusts and irregularities that are a feature of most winds near the earth’s surface. For all these reasons the acoustic qualities of the air are subject to marked variations, as everybody has observed.
Unusual audibility of distant sounds is a popular prognostic of rain. The fact underlying this belief is that when the air is full of moisture it is likely to be of uniform temperature, and therefore favorable for transmitting sound.
It is impossible to assign any limit to the distance at which loud sounds may occasionally be heard. No fact of nature has yet, so far as we know, matched Emerson’s metaphor of the “shot heard round the world,” but it is literally true that the sounds of eruption of Krakatoa, in August, 1883, were heard, like the roar of distant heavy guns, in the island of Rodriguez, in the Indian Ocean, 3,000 miles from the volcano. Moreover, the atmospheric waves set up by this outburst actually made the circuit of the globe, not only once, but at least three times, and the successive journeys were registered by barometers, if not detected by human ears. During the World War gun firing in Flanders was very commonly heard at places in England 140 to 150 miles distant. Several observers also reported that pheasants appeared, from their disturbed behavior, to hear cannonading over the North Sea that was beyond the range of the human ear.