DR. S. CHAPMAN ON THE DIURNAL VARIATIONS OF THE 



expedited if various observatories would undertake the reduction of their own data 

 on a unif'Tiii plan, and it is partly in the hope that some may be induced to co-operate 

 in this work that the present preliminary paper has been written. 



4. When determined from the mean of a number of whole lunations, the lunar 

 diurnal variation is found to be always of the same character, for every element and 

 at every station : it consists solely of a very regular semi-diurnal oscillation. Other 

 harmonic components of relatively small amplitude may be present, but their lack of 

 regularity and consistency proves them to be accidental inequalities which are no real 

 part of the phenomenon. This simplicity makes it probable that the lunar diurnal 

 variation will be easier to explain than the solar diurnal variation. 



SCHUSTER'S theory of the latter naturally suggests that the former is due to the 

 lunar tidal oscillations of the atmosphere. These oscillations have very little effect 

 upon the barometer, the ordinary diurnal barometric variation being a thermal and 

 not a tidal effect ; but a lunar barometric tide does exist, and has been evaluated 

 with a considerable degree of accuracy at some tropical stations (St. Helena, 

 Singapore, and Batavia).* The explanation gains weight from the fact that at 

 perigee the lunar magnetic variations are of distinctly greater amplitude than at 

 apogee,t and there is some evidence that the ratio of the amplitudes at the two 

 seasons is that which would be predicted by the tidal theory (1'23), though the 

 observational results do not suffice, as yet, to establish this definitely. 



5. Dr. VAN BEMMELEN, at Batavia, has recently collected all the existing 

 determinations of the lunar magnetic variation for different stations, and has 

 examined this material, together with newly computed data for other stations, to see 

 whether the magnetic field which produces these effects has a potential, and whether 

 the latter has its source above or below the earth's surface.J He finds that most of 

 the field, at any rate, has a potential, and that this arises partly above and partly 

 below the earth's surface, but that the internal field is too great to be merely a 

 secondary induction effect. This result should be accepted with some reserve, at 

 present, not only on account of the imperfections of the data, but also because the 

 seasonal change of the variations was disregarded ; in certain elements at some 

 stations the summer and winter variations are of opposite sign, and this renders it 

 unsafe to take the mean variation for the whole year. At many stations, 

 unfortunately, the data so far computed apply only to the whole year, so that if this 

 material was to be used, no course was possible save to adopt the mean of the year 

 for all One important result of VAN BEMMELEN'S work was to show that the 

 principal term in the potential of the lunar variation field was of the form Q 3 3 (in the 

 usual language of harmonic analysis, a tesseral harmonic of the second kind and third 



* SABINE, ' Phil. Trans.,' 1847 ; ' Batavian Observations,' 28 (1905). 



t See ' Trevandrum Observations' (BROUN), vol. I., p. 137, and SABINE'S and FIGEE'S discussions 

 already cited 



t ' Met. Zeitschr., 1 May, 1912. 



