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very small elevation, therefore, depart faster and faster from the 
surface. It follows from this, that audibility at greater distances is 
only possible, if the source of sound lies at some height above the 
surface and rays oecur which start with a negative elevation. The 
necessity of placing speakers in open air meetings on a platform with 
a soundboard above their heads, depends on the same phenomena 
of course. 
If then the audibility at great distances on October 8th is not to 
be attributed to the wind, how is the wide-spread first area of 
audibility to be explained ? 
The answer to this question is given by the ascensions of cable- 
balloons at Soesterberg on October 7th and 8th. On the 7th a very 
strong inversion was met with at 1600—2200 M. height, a rise 
indeed from O°.4 to 9°.9 C; on the 8th the inversion lay a 
little higher, 2500—3000 M. and had decreased to 3.8 — 10.8 or 
a little lower. 
The masses of air, which were above Soesterberg on the 7th, 
moved southward with a small velocity, some 2 M. per second or 
7 KM. per hour. From a comparison with the observations at 
Friedrichshafen and Hamburg and with the distribution of temperature, 
it appears certain enough, that the southerly motion extended to 
Belgium and lasted for a considerable time. Thus in 24 hours a 
distance of 168 KM. would have been traversed, so that those 
masses of air found themselves then above Belgium. 
There is every reason to assume, that especially in the early 
hours of the morning, when the surface temperature fell very low, 
the temperature at 2000 M. was much higher than that at the 
surface, and at noon only a little lower. Under these circumstances 
in the morning sound rays with an elevation even of 9° might be 
bent back to the earth at the limit of the inversion layer: if we 
assume reflection for simplicity’s sake, they would reach the earth 
again at 12 KM. distance. Rays with a smaller elevation would 
return at a greater distance, and this might be repeated several 
times, so that the rays undulated between the earth and the inver- 
sionlayer until either the energy was exhausted or the inversion 
layer ceased or decreased and the rays ascended into space. It is 
reasonable to assume, that with the very large angles of incidence, 
oceurring at the layer of discontinuity a great proportion of the 
energy of sound fell on the ‘reflected’? rays, the more so because 
the sky was everywhere heavily clonded or overcast on October 
7th and 8th in our country. 
Summing up, we may say that the meteorological circumstances 
