PLANETARy INFLUENCE ON THE EARTH’S MAGNETISM. 379 
and as second approximations, that is excluding terms involving i 2 , 
_p—_. 50089; &= + -86829; +1*01028 ; 
£,= -•00328; p 3 = -1-02186; &= +’00416; 
the degree of approximation is in the second case close, and in the first almost perfect. 
IV. 
Application of the processes described to determine whether or not there be any 
periodical variation of disturbances of Magnetic Declination and Horizontal 
Force at Bombay , due to the influence of the planets Mercury , Venus, and the 
Earth, in the periods of their respective orbital revolutions* , and of Mercury, 
Venus, and Jupiter in their synodic periods. 
15. In view of the remarkably definite evidence of periodicity in sun-spots indicative 
of planetary influence, brought to light by the investigations of Messrs. De La Rue, 
Stewart, and Loewy, and having regard to the common subjection of sun-spots and 
terrestrial magnetism to the well-known decennial period, it seemed to the writers very 
desirable to examine whether a similar connexion was exhibited by the two phenomena 
in respect of the planetary periods. The connexion was first shown to exist, by General 
Sir Edward Sabine, between the larger disturbances of terrestrial magnetism and sun- 
spots, but it has since been extended to include also the regular magnetic variations. 
The present inquiry will, however, be confined to the larger disturbances, and of these 
to the disturbances of Magnetic Declination and Horizontal Force at Bombay, of which 
a large body, extending over a period of twenty-six years, is available for use in the 
discussion. 
16. A description of the Declinometer, and of the method adopted for separating dis- 
turbances, which is that of General Sabine, appears in the 4 Philosophical Transactions,’ 
1869, pp. 363 to 368, and, like the Declinometer, the Horizontal-force Magnetometer is 
of the kind by Grubb of Dublin, originally supplied to the British Colonial Observa- 
tories. Disturbed observations of Declination (Easterly) may be defined as all those 
observations which give a value of the easterly declination in excess of the average of 
the remaining observations at the same hour during the same month by more than l'-4, 
and the easterly disturbance is that excess ; and disturbed observations of Declination 
(Westerly) are all those observations which give a value of easterly declination in defect 
of that average by more than l'-4, and the westerly disturbance is that defect. In 
Table I. the aggregates of such excesses and defects are shown for each month in each 
of the twenty-six years from 1847 to 1872. Disturbed observations increasing the Hori- 
zontal Force are all those which give a value of the Horizontal Force in excess of the 
average of the remaining observations at the same hour during the same month by more 
* The mathematical expression for the Earth’s influence being analogous to the expressions for the influence 
of Mercury and Yenus, the influences are here classed together indiscriminately, although doubtless they are 
not wholly of the same character in each case. 
