THE ERUPTION OF KRAKATOA IN 1883 235 



Air Waves 



The air waves generated by the eruption were of three kinds: 

 first, those sufficiently rapid to give rise to sound ; second, larger waves 

 that caused the breaking of windows and the cracking of walls, a 

 hundred miles away, at Batavia; and third, those of still greater dimen- 

 sions that travelled several times around the earth. 



The sounds caused by the explosions were heard at Ceylon to the 

 northwest ; Perth and other stations in Australia- to the southeast, and 

 at Manila to the northeast. These places are situated close to a circle 

 drawn from Krakatoa as a center with a radius of 30 degrees. To 

 the west the sounds were carried by the trade-winds a much greater 

 distance, and were heard at the island of Rodriguez, nearly 3000 

 English miles from the scene of eruption. This is interesting not only 

 as being the most remote point at which the reports of the explosions 

 were noticed, but as "the only instance on record of sounds having 

 been heard at anything like so great a distance from the place of their 

 origin." The area over which the sounds were recorded is about one- 

 thirteenth of the earth's surface. 



The detonations in the vicinity of the volcano, although very vio- 

 lent, were not as loud as might be inferred from the distance at which 

 they were heard, for the great rain of pumice and the heavy cloud of 

 dust seem to have acted as a curtain through which the sound waves 

 were unable to penetrate. At some places in the immediate neighbor- 

 hood of the volcano sounds ceased to be heard after ten o'clock on the 

 morning of August 27; although it is known that the explosions 

 continued with great intensity after that time. 



The great air wave that owed its origin to one of the last paroxysmal 

 outbursts of Krakatoa was recorded on the barograms at the observa- 

 tories scattered over the earth. Theoretically an air wave caused by a 

 sudden explosion would spread in a circle around the earth till it 

 reached a point 90 degrees from its origin ; it would then travel onward, 

 contracting as it advanced, till at the antipodes it would come to a 

 focus and thence be reflected back to its starting-point, from whence it 

 would once more travel to the antipodes and so on, till it gradually 

 became lost. The barograms that have been collected show that the 

 great wave passed seven times round the earth, four times from 

 Krakatoa to the antipodes, and three times from the antipodes to 



