76 



DISCUSSION OF THE INFLUENCE OF THE MOON 



According to Broun, then' is al Makerstoun a maximum horizontal force at the 

 time of the moon's greatest north and south declination, with a minimum force at 

 the time of her crossing the equator; in two cases, therefore, viz: for north decli- 

 nation and no declination, the Makerstoun and Philadelphia results agree; while 



in the third case they disagree or remain doubtful. Kreil's results, from the Prague 

 observations, do not appear to me sufficiently decisive and regular to admit of com- 

 parison. 



Influence of the Moon's Variation in Distance on tin Horizontal Force. — By a 

 process of reduction similar to that followed in the preceding investigation we 

 find :— 



Table XIII. 



One day before perigee 

 On the day of " 

 One day after " 



One day before apogee 

 On the day of " 



One day after 



Scale divisions. 

 —1.5 

 —1.9 



—2.11 



+2.3 

 +2.3 



+2.7 



I Mean 



■ .1 

 -1.8. 



Mean —2.4. 



The probable error of any one result is about the same as in the preceding re- 

 sults (Tables XI. and XII.). The results for variation in the moon's distance are 

 more consistent and satisfactory than those depending on the phases and declination 

 changes. The lunar effect is to diminish the horizontal force by its 0.000066 part 

 in perigee, and to increase it by its 0.000088 part when she is in apogee. 



The Prague results arc the same, viz: a greater horizontal force at and after the 

 moon's apogee than at and after her perigee; a three years' series of observations 

 at Milan, however, do not agree therewith. 



In no branch of magnetic research would additional results from independent 

 observations, particularly al stations widely apart, be more acceptable and valuable 

 than in the study of the lunar effect in its various manifestations. 



I'l BLISHED B1 THE SMITHSONIAN INSTITUTION, 

 WASHINGTON CI T Y . 



Nov k si n E r. . IS 6 2 . 



