lean speed of 
propagation was about 700 miles an hour, less by 23 miles 
than the velocity of sound at zero Fahrenheit ; the velocity, in 
fact, seems to have corresponded to that of sound in air at 20 
or 30 degrees below zero. 
Among other interesting observations noted in connection 
with this eruption are those relating to the propagation of sound. 
Authentic instances are recorded of sounds caused by the ex- 
plosions being heard at distances of 12 10, 1902, 2014, 2267^ 
and 2968 miles, being quite the longest distances that sound 
has been known to travel. The English Report includes a 
number of interesting and instructive hypotheses. The con- 
census of opinion as to the red sunsets which formed so con- 
spicuous a feature of the autumn evenings of 1883 is that they 
may be traced to Krakatoa. The general conclusions are 
admirably traced by Sir Robert Ball : 
First of all it would be natural to ask whether the existence 
of volcanic dust in the air could have produced the optical 
effects that have been observed. This must be answered in the 
affirmative. Then it would be proper to inquire whether other 
volcanic outbreaks in other parts of the world, and on other 
occasions, had been known to have been followed by similar 
results. Here, again, we have page* after page of carefully 
stated and striking facts which answer this question also in the 
affirmative. Next it would be right to see whether the se- 
quence in which the phenomena were produced at different 
places in the autumn of 1883, tallied with the supposition that 
they all diverged from Krakatoa. The instances that could be 
produced in support of the affirmative number many hundreds, 
though it must be admitted that there are some few cases 
about which there are difficulties. Surely we have here what 
is practically a demonstration. It is certain that these optical 
phenomena existed. No cause can be assigned for them ex- 
cept the presence, at that particular time, of vast volumes of 
dust in the air. What brought that dust into the air except 
the explosion of Krakatoa? Most people find themselves un- 
able to share the scruples of those who think there can be a 
doubt on the matter. Would another eruption of Krakatoa, 
followed by a repitition of all the optical phenomena, convince 
them that in this case, at all events, post hoc was propter hoc. 
Perhaps not, if they have already failed in being convinced by 
the fact that, when Krakatoa exploded two centuries ago, blood 
red skies appear to have been seen shortly afterwards as far 
away as Denmark. 
