Of acoustic phenomena that belong especially to the domain of meteorology, probably thunder is the one that excites most general interest. The sudden expansion of the air along the path of a lightning discharge, due partly but probably not entirely to the heat generated, appears to be an adequate explanation of the explosive sound of thunder, though somewhat different explanations have been suggested. If the discharge is near at hand, we generally hear a single loud crash. More distant lightning is usually attended by rumbling. The common and obvious explanation of rumbling is that it is due to the arrival of the sound progressively from different points along the path of discharge, which may be a mile or more in length. A crooked path would account for reenforcements and diminutions of the sound. Another cause of irregularities in the sound is probably “interference” (combinations of waves that tend either to strengthen or to neutralize each other), especially in the case of multiple lightning discharges, such as we have described elsewhere. Lastly, thunder is further complicated by echoes from the ground and probably also from the air (not exclusively from clouds), though much uncertainty prevails concerning these aerial echoes. The sounds of thunder have been the subject of some interesting investigations on the part of an Austrian meteorologist, Dr. Wilhelm Schmidt, who has devised apparatus for making an automatic registration of the sound waves that constitute a thunderclap. He finds that there is a great preponderance of waves of very long period, including many of too low a pitch to be audible, though perceptible through the rattling of windowpanes, etc. In fact, the greater part of the energy involved is represented by these long, inaudible waves, so that one really _hears_ only a small part of a clap of thunder. The statement has often been made, on the authority of Humboldt, that thunder is never heard at sea, at any point far from land. This matter was investigated by the magnetic survey yacht _Carnegie_ during a long cruise in the Pacific in 1915. Of twenty-two displays of lightning, six were accompanied by thunder.