46 
DR. S. CHAPMAN ON THE SOLAR AND LUNAR 
lunar diurnal term is variable and irregular, while the semi-diurnal term is constant, 
its value (calculated from forty years’ observations) being, in millimetres of mercury, 
(36) 0’063 sin (2f + 65°). 
Wagner (‘Gottingen Abh.,’ ix., 4, 1913), has also discussed six years’ hourly 
barometric observations at Samoa, with the aim of determining the various charac¬ 
teristics of the lunar semi-diurnal barometric variation. The material was insufficient 
for the attempted purpose of investigating the effect of season, lunar distance, 
declination, and phase, but the mean result for the semi-diurnal tide may be 
quoted, viz., 
(37) 0‘039 sin (2^ + 33°). 
The data do not suffice to determine the dependence of phase and amplitude on 
latitude, but we shall assume that the phase is constant, while the amplitude 
is specified by the function Q 2 2 . The Samoan result does not support this conclusion 
very strongly when compared with the Batavian determination, but the material 
is insufficient to enable a definite judgment to be made as yet. For the present the 
Batavian result will be adopted as the basis for discussion in this paper. The 
corresponding value of the velocity potential T> is given by 
(38) ffi = 32'4B . O'OIOQ/ cos (2£ + 65°). 
§ 21. The Electrical Conductivity of the Upper Atmosphere. 
It has already been mentioned (§ 6) that the electrical conductivity of the upper 
atmosphere was discussed by Schuster in his second memoir. The possibility of the 
production of a conducting layer such as was suggested in that discussion by the 
agency of ultra-violet radiation from the sun has recently been considered by Swann, # 
in connection with recent physical data bearing on the problem. Assuming the 
ionized constituent of the atmosphere to be oxygen, it is possible to determine the 
rate of supply of energy of ultra-violet radiation necessary to maintain the proposed 
conductivity (10 -13 C.G.S. electro-magnetic units, in a layer 300 km. thick, where 
the average pressure is 10~ 6 atmosphere). • The ionization potential for oxygen -is 
9 volts, and only the radiation of wave-length less than A 1350 is available for 
ionization.t Considering the solar spectrum to be that of a black body at 6000° C., 
it appears that 1‘6 . 10 _a of the whole solar radiation would be thus available; it is 
found, however, that even if the total radiation of all wave-lengths were absorbed in 
the act of ionization, the rate of ionization would still be only one-sixteenth of 
what is required. Swann points out that the simplest method of overcoming the 
* Swann, ‘Terrestrial Magnetism,’ XXL, p. 1, 1916. 
t Hughes, ‘ Proc. Camb. Phil. Soc.,’ 15, p. 483, 1910; ‘Phil. Mag.,’ 25, p. 685, 1913. 
