of wind towards the Ci-level played a certain part cannot be denied, 
but it is probable, that this influence made the rays return to the 
surface at a distance greater than 160 km. Especially numerous 
are the reports near the circle of 200 km., as was observed every 
time in the case of the naval guns. 
November 15. There is a rapid increase of wind upwards, but 
always in a SE-ly direction, hence perpendicular to the direction 
from the battlefields towards our country, so that for this direction 
no influence was to be expected on the bending back of the sound 
towards the surface. For a propagation from the 8. and still more 
from the SE. certainly some influence was to be expected, and it 
is possible that the rather confused picture, which the reports furnish 
on this day, ought to be ascribed to war noises from much more 
southerly quarters, whieh were carried to our country this time by 
the strong upper wind, e.g. from the Argonne. The inversions, 
extraordinarily large at Lindenberg, must have been smaller over 
our country on account of the higher surface-temperature. 
January 24%. The inversion at Lindenberg is important enough; 
on account of the Sunday (October 18 and November 1st were Sun- 
days likewise) no observation from our country is available. Considering 
the position of the high pressure, the supposition that the inversion 
was present likewise above the North Sea and perhaps more intense, 
is not risky. De Bilt, Helder and Flushing had an overcast sky all day 
and the temperature only rose to 2°, resp. 3°5 and 8°. Above the layer 
of fog probably the same or a slightly higher temperature will have 
reigned. Hence it is possible, that rays but slightly inclined reached 
the inversion layer above the northern part of Groningen, and were 
bent back there, aided by an increasing North-component of the wind. 
We therefore leave undecided, whether the weak sounds, at the place 
where otherwise the silent region would have been, must be ascribed 
to altogether horizontal propagation or to rays of little elevation 
bent back. The heavy sounds however at 170—200 KM., which 
emerge there suddenly enough, can be explained hardly otherwise 
than by propagation through the higher atmosphere. 
Conclusions. 
1. In the literature of these last few years a number of cases have 
been described, in which according to careful investigations in the 
propagation of heavy sounds through the Rn region” 
occurs. With the roar of cannons during the siege of Antwerp 
(October 7%—9th 1914) such a silent region is displayed in an extra- 
ordinarily regular form, whereas on a number of other days of heavy 
‘ 
a 
ep) 
Proceedings Royal Acad. Amsterdam. Vol. XVIII. 
