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. Writers have often commented on the fact that thunder cannot be heard so far as the sounds of artillery. It has been affirmed that 10 miles or thereabouts is its maximum range of audibility. As a matter of fact, however, thunder has occasionally been heard at much greater distances, up to 20 or 30 miles; but it remains true that the distance is always much less than that at which loud terrestrial sounds are audible. The reasons why this should be so are not far to seek. In the first place, the intensity of a sound depends upon the density of the air in which it is generated, and not upon that of the air in which it is heard. The air, as we know, diminishes in density upward. Balloonists thousands of feet above the earth hear with remarkable clearness sounds from the ground below, but people on the ground cannot hear similar sounds from the balloon. As thunder is mainly produced at the level of the clouds, it is subject to this peculiarity. Again, cannonading is heard at great distances only when the air is comparatively calm, and perhaps only when it is arranged in well-defined horizontal layers of such a character as to keep the sound from spreading far aloft. Very different conditions prevail during a thunderstorm; in fact the conditions are then just such as would scatter and dissipate the sound waves. Lastly, the noise of a cannon or the like comes from a single place and the energy of the disturbance is concentrated to produce a single system of sound waves; while the disturbance due to lightning is spread over the long path of the discharge. The audibility of sounds at abnormally great distances is not usually a matter of practical importance, but the converse phenomenon--the failure of sounds to carry to normal distances--has been responsible for a great number of marine disasters on such fog-ridden coasts as those of the British Isles, eastern Canada and California. Hence some of the ablest physicists of both the Old World and the New have tried to ascertain the conditions under which this phenomenon occurs.