298 On the Magnetic Effect of the Sun or Moon on Instruments. 

 2x 0-5236 x 7831 x 100877000 x 206264-8 



which is less than 0"-094, and 



2 x 0-5236 x 7-831 x 100877000 x 206264-8 



which is less than 0"-036. Hence at Gbttingen the direct dis- 

 turbance of the instrument of declination does not amount, at its 

 maximum, to one-tenth of a second of arc, and that of the dip 

 circle does not reach even one twenty-seventh of a second. 



Now the observations with which these should be compared 

 have been made at several stations. The principal part of the 

 observed lunar diurnal variation consists of a term depending 

 on twice the lunar hour-angle, but there is also a small term 

 containing the simple hour-angle. This latter is the one which, 

 as Dr. Lloyd has shown, the direct action of the moon would 

 affect, and General Sabine* has determined the following values 

 for its coefficient, in calculating the formulas which would best 

 represent the observations at the several stations : — 



-1-05 at Toronto, 

 + 0-88 at St. Helena, 

 + 1'21 at the Cape, 

 + 0-97 at Hobarton, 

 -0-81 at Pekin, and 

 -2-04 at Kew 

 for the declination ; and — 



u 



— 1-14 at Toronto, 



+ 1-32 at St. Helena, 



— 0*94 at the Cape, and 

 —0-48 at Hobarton 



for the inclination. 



There is then no ground for presuming, from the minuteness of 

 the coefficient, that the moon is not of as magnetic or even much 

 more magnetic materials than the earth. On the contrary, the 

 actual magnitudes of the coefficients are too large to be with 

 probability attributed solely to the direct effect of the moon, even 

 if it were not evident from other considerations, that some cause 

 acting by different laws has contributed the greater part to them. 



If the comparison with the earth be made mass for mass 

 instead of bulk for bulk, the above disturbances must be reduced 



* See p. cxlvi of the Introduction to the 2nd volume of the St. Helena 

 Observations. 



