INYESTIGATION 



INFLUENCE OP THE MOON ON THE MAGNETIC VEETICAL FORCE. 



The method of discussion of the lunar effect on the vertical component of the 

 magnetic force in no way differs from that employed for the horizontal component, 

 which latter has been explained in Part VI. 



The series of observations available for the lunar discussion extends from Febru- 

 ary, 1841, to June, 1845, inclusive. From February, 1841, to October, 1843, the 

 observations are bi-hourly; from October, 1843, to the end of the series they are 

 hourly. The record of May, 1841, is not quite complete, and in January, February, 

 and March, 1843, but one observation a day is recorded. As increasing numbers 

 denote a decrease of force, a positive sign of the tabulated differences between 

 monthly normals and each individual undisturbed reading (at the normal tempera- 

 ture) indicates a greater force than the normal value, a negative sign indicates the 

 reverse. 30 scale divisions being the limit beyond which difference an observation 

 has been considered as belonging to the class of disturbances, aU differences here 

 recorded are below this limit. One scale division is 0.000033 parts of the force. 

 The tabular numbers are expressed in scale divisions. 



In tracing out the lunar effect upon the vertical force we have to contend with 

 greater irregularities than was experienced in the case of the horizontal force. The 

 vertical force magnetometer is more subject to changes, and the correction for tem- 

 perature far exceeds that of the horizontal force. 



The total number of observations and differences formed in the inquiry of the 

 dependence of the force upon the moon's hour angle is 19513, which distribute 

 themselves over the months and years as follows : — 



Table I. — Number of 



Observations for Lunar Discussion. 





MontK. 



1841. 



1842. 



. 1843. 



1844. 



184.5. 



Sam. 



January 







207 







544 



611 



1362 



February 













239 



198 







549 



554 



1540 



March. 













257 



286 







502 



539 



1584 



April 













256 



255 



250 



512 



581 



18.54 



May 













207 



246 



288 



617 



602 



1960 



June 













219 



275 



296 



529 



571 



1890 



July 













258 



281 



308 



581 



— 



1428 



August 













283 



284 



314 



535 



— 



1416 



September 











267 



246 



292 



568 



— 



1373 



October . 











280 



298 



680 



607 



— 



1765 



November 











275 



299 



528 



596 



— 



1698 



December 











239 



304 



541 



559 



— 



1643 



Year 



2780 



3179 1 3397 



6699 



3458 



19513 



( 61 ) 



