326 IN STARRY REALMS. 



waves, and beautiful optical phenomena made the circuit 

 of the globe even more than once or twice. In these last 

 respects the eruption of Krakatoa is unique. 



Professor Judd has satisfactorily accounted for the 

 enormous manufacture of dust during the eruption. It 

 appears to consist of comminuted pumice, and is produced 

 by the attrition of the pumice masses, as in successive out- 

 bursts they are hurled aloft, and then tumble back again 

 into the crater. 



It appears to me that the most remarkable incident 

 connected with the eruption of Krakatoa was the pro- 

 duction of the great air-wave by that particular explosion 

 that occurred at ten o'clock on the morning of Monday, 

 August 27. The great air-wave was truly of cosmical 

 importance, affecting as it did every particle of the atmo- 

 sphere on our globe. This phenomenon alone extends the 

 study of Krakatoa beyond the province of vulcanology, 

 and gives to the subject a particular interest in physical 

 science. 



A pebble tossed into a pond of unruffled water gives 

 rise to a beautiful series of circular waves that gradually 

 expand and ultimately become evanescent. A very large 

 body falling into the ocean would originate waves that 

 might diverge for miles from the centre of disturbance ere 

 they became inappreciable. "Waves can originate in air 

 as well as in water. We are not at this moment speaking 

 of those familiar air- waves by which sounds are conveyed. 

 The waves we now mean are inaudible and apparently 

 much longer undulations than those of sound. 



Imagine a great globe, which for simplicity we may 

 think of as smooth all over. Let us suppose that this 



