1865.] 2Q3 [Chaee. 



apparent, and may result from the action of a uniform law. I be- 

 lieve that I have now given the needed explanation, and since I 

 have shown experimentally that the phenomena are such as should 

 be produced by gravitation, it is reasonable to assume that they 

 probably are so produced. The probability is increased by the dis- 

 appearance of the night oscillation in summer (V), the probability 

 that when it is observed, it results from thermal disturbances, and 

 the greater stability of those critical hours which are nearest to the 

 hours of maximum sunward gravitation (VI). 



The precise coincidence, both in time and direction, of the lunar- 

 diurnal declination and tidal curves (VII), the unavoidable inference 

 that the moon has no constant or specific magnetic action (VIII), 

 the "establishment" of ten minutes at the Philadelphia station (IX), 

 the correspondence of the lunar and solar curves in the diminished 

 winter amplitude (X), and the uniformity at all stations of the semi- 

 annual variation curves (XI), are all necessary corollaries of my 

 hypotheses. 



The demonstration of a connection between the daily and annual 

 variations of magnetism and gravitation would be incomplete, if our 

 theory could not be so extended as to explain the decennial and secular 

 changes. For such an extension we are compelled to wait until fur- 

 ther study and observation have more precisely defined the character 

 and value of those changes, and suggested all the important gravita- 

 tion disturbances of long period to which they may be plausibly re- 

 ferred. We may find, however, in the attraction of Jupiter, one of 

 the possible causes of the 10-11 year period, while nutation, pre- 

 cession,* geological upheaval and depression, change of seasons, accu- 

 mulation or diminution of polar ice, and the shifting position of the 

 centre of gravity of our planetary system, must all necessarily con- 

 tribute to the production of gradual changes in the terrestrial gravita- 

 tion currents. The belief does not, therefore, seem unreasonable, 

 that the feeble vibrations of the tremulous needle may not only fur- 

 nish us with a delicate scale for weighing (as we have already ap- 

 proximately done), the huge mass of the sun, but they may also aid 

 us in the discovery and verification of other important cosmical phe- 

 nomena, and the assignment of their appropriate laws. 



Although it is probable that terrestrial magnetism is mainly owing 

 to currents circulating above the surface, it is well known that there 



* Hansteen supposed that the motion of the magnetic poles was con- 

 nected with the precession of the equinoxes. 



