270 SECTIONAL TRANSACTIONS.—A. 
(1) Extraneous ionising agencies (e.g. solar corpuscles and terrestrial 
thunderstorms). 
(2) Horizontal motion of ionisation from more densely ionised regions by 
winds or diffusion. (The coefficient of lateral diffusion may be shown 
to be a maximum at about 100 km.) 
(3) Readjustment of ionisation already present due to tidal or thermal 
influence, bringing about a sharper gradient of refractive index at a 
particular level and thus giving rise to quasi-reflection as distinct from 
the normal deviating process. 
(6) The maximum ionisation content in Region F is often found to 
increase during the night. Possible influences to be considered are : 
(1) A nocturnal ionising agency. 
(2) Readjustment of ionisation distribution due to cooling and shrinkage 
of the atmosphere causing an increased electron concentration. 
(c) Very occasionally there are found subsidiary regions of ionisation 
(1) between Regions E and F, and (2) above the main Region F. These 
have been noted at both the Slough Radio Research Station and at the Halley- 
Stewart Laboratory, Hampstead. 
Freak wireless transmissions —A careful watch was now being kept each 
day on what might be called the weather conditions in the ionosphere. 
Such work was being carried out at the Slough Radio Research Station and 
at the Halley-Stewart Laboratory at Hampstead. As often happened in 
the study of geophysical phenomena, abnormal events proved of special 
interest and importance. Both of the ionised regions were found to exhibit, 
occasionally, increases of ionisation even at night, when ultra-violet light 
from the sun could not possibly be reaching the upper atmosphere. 
In 1930, during some experiments carried out at King’s College, London, 
curious increases of ionisation were noted in the lower of the two regions 
(the so-called Kennelly-Heaviside Layer). There appeared to be some 
influence maintaining and even increasing the ionisation which normally 
decreased during the night. The same effect had since been noticed in 
different parts of the world. He wished to put forward the theory that this 
abnormal ionisation, which he had found gave almost mirror-like reflection 
of the waves, might be responsible for the freak transmissions which had 
been noted from time to time in long-distance transmission. 
It had also been found that there was a fairly definite limit in the short 
wave-length range below which one could get only a quasi-optical range. 
Waves shorter than the limiting value (8 to 10 m.) usually pierce the 
ionosphere and leave the earth altogether. But calculation showed that 
under the abnormal conditions mentioned, which appeared to be connected 
in some way with both thunderstorms and magnetic storms, the limiting 
wave-length should be less than its usual value. 
Mr. J. A. RATCLIFFE. 
Collisions between electrons and neutral molecules cause absorption of 
a wave travelling through the ionosphere. From observation of the 
resultant absorption deductions may be made about the frequency of 
collision of the electrons. 
Calculation shows that a region of absorption may be situated below the 
region of deviation of the wave, the extent of the absorbing region being 
determined by the height distribution of the electron collisional frequency. 
‘There is no need to postulate a lower ‘ layer’ of ionisation to explain this 
