ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 309 

 PROPAGATION OF SOUND IN AIR. 



The rate of propagation of sound in air is believed, on theoretical, 

 grounds, to increase in some slight proportion with the intensity of the 

 sound. The mean velocity of the explosion sounds and air waves of 

 Krakatoa, in the eruption of 1883, was about 700 miles an hour, or less 

 by about 23 miles than the velocity calculated for sound in air at 0° F.; 

 it corresponded with the theoretic velocity at between —20° and —30° F. 

 How was the rate affected by the temperature of the upper air, and what 

 mean value of temperature can be assumed in that total propagation 1 ? 

 The rate of movement diminished in the second and third circuits of 

 this great air wave round the globe; the rate for the first passage in 

 one direction was 10.23 per hour; for the last, 9.77 per hour; in the 

 other direction, 10.47 and 10.27, respectively; so that a diminution of 

 rate with diminishing intensity does seem to have occurred. The high 

 temperature of the tropics does not appear to have raised the rate, as 

 might be expected, above the rate in the temperate zones. Nor did the 

 air wave travel faster, so far as can be deduced, than ordinary sound, 

 although, considered as a very low note, it might theoretically be 

 expected to do so. The velocity of the wave in the tropics toward the 

 east was retarded ; in the extratropics toward the west was retarded 

 toward the east accelerated; from the data available in the report of 

 the Krakatoa Committee of the Eoyal Society of London it appears 

 that in the tropics there was an excess ot general movement of air from 

 east to west of about 14 miles an hour, and in the extratropics an 

 excess of 14 miles from west to east. Thus the propagation of the air 

 waves throws some light on the mean air movement within and without 

 the tropics. The effect of cold in the regions both of the South and 

 North Poles was not what might have been expected; there was no 

 discoverable retardation by the low temperature. All these results 

 have yet to be interpreted, but may perhaps themselves contribute 

 toward a better knowledge of the laws of the transmission of sound 

 and great waves in air. 



The sounds of Krakatoa, which were audible over an area exceeding 

 twice that of Europe, were not very loud in some places in the imme- 

 diate neighborhood of the volcano. It seems as if the mass of falling 

 ashes, pumice, mud, etc., and the great variations of temperature and 

 humidity in the midst of the hot materials must have exerted a pow- 

 erful dulling effect. Striae or laminae of alternate hot and cold air 

 seem to be very capable of diverting and reflecting sound waves. 



With regard to the conveyance of ordinary sounds in air in various 

 kinds of weather, Professor Tyndall and others have arrived at certain 

 results of much scientific interest and practical importance. The 

 condition of the air varies very greatly with regard to transmission of 

 sound, and often without any apparent cause. Fog, rain, hail, and 

 snow do not sensibly diminish sound. The most powerful cause of 



