EARTHS MAGNETISM PRODUCED BY THE MOOT* AND SUN. 281 



magnetic diurnal variations. The calculations are given in Part II., in a very general 

 funn; the work is in some respects simpler and more direct than in SCHUSTER'S 

 investigation, owing to the adnptinn of the resistivity, instead of the conductivity, 

 as the variable. The formal results (which as yet, however, are at a somewhat 

 incomplete stage) are reduced to numerical form and compared with the observed 

 data. The whole of the discussion is collected in Part I., and it is shown that the 

 fourth harmonic component of the lunar variation favours the assumption that the 

 atmospheric conductivity may fall to a very small value during the night hours. 

 The question of the seasonal variations, as affecting both the solar and lunar effects, 

 is barely touched on, since though it arises naturally from the calculations in Part II., 

 better observational material is necessary to reali/e the proper use of the theoretical 

 work. A fuller discussion is reserved therefore till the new determination of the 

 potential of the solar variation, already mentioned, is completed. 



I'AliT I. 



3. The magnetic elements show regular periodic changes depending on the lunar 

 hour angle,' just as on the solar hour angle : the latter variations are considerably the 

 greater of the two, and .almost entirely mask the lunar variations. KREIL,* of 

 Prague, in 1841, first established the existence of these changes, and since then a 

 very limited number of investigators! have confirmed and extended KREIL'S 

 discovery. Owing to the nearly equal length of the solar and lunar days, the 

 separation of the two effects involves considerable rearrangement of the observed 

 data as usually tabulated, and the smallness of the lunar variation renders it 

 necessary to deal with a large quantity of material in order to eliminate accidental 

 errors. The determination of the lunar diurnal variation for the three magnetic 

 elements at a single station is therefore a laborious undertaking, and hardly any 

 observatory, as yet, includes such an examination of its observations in its scheme of 

 work. If the potential of the magnetic field producing these variations is to be 

 found, however, they must be computed not merely for one, but for several stations, 

 well distributed on the earth's surface. This formidable task would be much 



* Bohemian Society of Sciences, 1841. 



t BROUN, 'Trevandrum Observations,' I., 1874; CHAMBERS, 'Phil. Trans.,' A, voL 178, p. 1 (1887) ; 

 ' Batavian Observations," BERGSMA and VAN DBR STOK, vols. I., III., IX., X., XVI., also Proc. Roy. 

 Acad.,' Amsterdam, IV., 1887, and ' Archives Nderlandaises," XVI. ; FIGEE, ' Batavian Observations,' 

 XXVI., 1903; LAMONT, 'Sitz. d. K. Akad. d. Wiss., 1 1864, t. 11, 2, Munich. SABINK, 'Phil. Trans.,' 

 1863, 1856, 1857 ; 'Roy. Soc. Proc.,' X., 1859-1860. 



Also the published ol>scrvations at St. Helena, Toronto, Hobarton, and Cape of Good Hope (edited by 

 SAIIINK), and at Melbourne, Dublin, and Philadelphia. Also AIRY, 'Greenwich Observations,' 1859 

 and 1867. 



Also Moos, ' Bombay Magnetic Observations,' 1846-1905, vol. II. (1910) ; and VAN BEMMEI.EN, ' Met. 

 Zoitschr.,' May, 1912. 



VOL. COXIII. - A. 2 O 



