SECTIONAL TRANSACTIONS.—A. 271 
region of absorption. ‘The existence of such an absorbing region is required 
to explain the absorption observed at different distances from a transmitter. 
Absorption of a wave near the top of its trajectory is related to the group 
retardation there, and from a comparison of the observed magnitudes of 
these quantities we deduce that the collisional frequency in the F region is 
about 5 X 10° per second, and in the E region is about 2 < 104 per second. 
The extra group retardation of the ordinary wave on a wave-length of 
60 m. in the day-time perhaps explains why Eckersley ? found it to be 
weaker than the extraordinary wave. 
To explain some unexpected results it has been suggested * that absorption 
determines the greatest frequency which may be reflected from the F region 
at midday in summer, whereas in winter the maximum electron density 
determines this critical frequency. If this is the case, then the temperature 
of the F region at a summer midday must be considerably greater than that 
at a winter midday. 
Automatic records have been taken showing how the height of reflection 
of wireless waves of a fixed frequency varies with the time of day. These 
records show the occurrence of intermittent reflections from a height of 
about 105 km.—that is, below the ordinary E region. Such reflections 
may occur by night or by day. They are presumably due to some ‘abnormal’ 
source of ionisation. A statistical analysis indicates that they are probably 
related to the occurrence of (a) magnetic storms, and (6) thunderstorms. 
The opportunity of observing on a series of different frequencies in rapid 
succession occurred recently, during a thunder shower. During the 
shower wave-lengths down to 45 m. were reflected (partially) from a 
height of 105 km., whereas half an hour before and a quarter of an hour 
afterwards there were no reflections from regions below 250 km. (F region) 
on any wave-length shorter than 75 m. It appears as though the thunder 
shower had produced ionisation at a height of about 105 km. 
Mr. R. NatsmitH.—The polar ionosphere. 
It has been shown by Appleton that the main ionising agency for the 
E and F regions of the ionosphere in temperate latitudes is the ultra-violet 
light from the sun. 
It has also been suggested by Chapman that charged particles emitted 
from the sun may produce ionisation in the upper atmosphere, and the 
phenomenon of the aurora appears to confirm this theory. 
Observations made by the British Wireless Expedition during the second 
International Polar Year are examined with regard to these two theories. 
The first of these theories is examined with reference to the whole year’s 
observations, but more particularly under the special condition existing in 
the Polar regions in the winter when no ultra-violet light from the sun is 
reaching the earth, and in the summer during the period of the midnight 
sun. 
The maximum ionisation effects of charged particles are to be expected 
in northern latitudes. This theory is also examined with reference to the 
year’s observations, but more particularly during periods of magnetic 
disturbance. 
There is at present no unanimity of opinion on the relative importance of 
these two influences, but the present series of observations appear to indicate 
that both are necessary. 
1 Proc. Roy. Soc., A, 115, 291 (1927). 2 [bid., A, 141, 710 (1933). 
§ Proc. I.R.E., 22, 499 (1934). 
