be confined to at most the difference 2 >< 10 (tg. 23° 49’ — tg. 23°3’), 
that is about 0.82 KM. 
Variations in the fall of temperature, even considerable inversions, 
have neither a great influence on these steep rays, the more so, 
because the mean fall of temperature over 10 KM. seldom undergoes 
great variations. E.g. a sudden rise of temperature of 10°, which 
was not balanced afterwards, would at 5 KM. increase the velocity 
of sound in the proportion 1: V5, which would alter the elevation 
by 0°30’. In other words: for these steep sound rays even the most 
heavily disturbed air behaves as a bad window-pane for a perpen- 
dieularly incident ray: it goes through almost undisturbed. Hence the 
sound, refracted by the high strata of the atmosphere, always returns to 
the earth at practically the same distance; regular modifications of the 
sound-limit, which are connected with the regular and pretty great 
velocities, which may oecur in the higher atmosphere, will always 
show the same features, since we may assume at these considerable 
heights a practically constant state of motion. 
§ 9. From this we may immediately infer, that the singularly 
shaped silent regions, observed in many of the cases quoted in the 
introduction, cannot be ascribed to a co-operation of the refraction 
in the high atmosphere and the disturbances of the lower atmosphere, 
but ought to be explained by the latter alone. That in these and 
other cases the regular second region of audibility does not appear, 
will often be explained sufficiently by the fact, that this sound is 
rather weak, absolutely speaking, and may easily be covered by sounds 
transferred in another way. Moreover we have already indicated 
the possibility, that originaily little sound is emitted in certain 
definite directions. In the case of volcanic eruptions the emitted hot 
gases and vapours, mixed with ashes, may cause tremendous disturb- 
ances in the distribution of windvelocity and temperature. Yet it 
seems to us, that among these cases some indications are found of 
an increased audibility at distances of the order of 140—160 KM. 
a. In the case of the dynamite-explosion at the Eigerwand (2 
near the Bodensee, distance exactly 160 KM. 
b. December 7th 1909 Asama 170 K.M. (Fig. 3) 
April 4th 1911 AD cu. GEIS. 45) 
55 rodel ELS ON 
December 25th 1910 i 145 „ (Fig. 4) 
2nd 1912 A One 
2, 
