ON THE MAGNETIC HORIZONTAL FORCE. 



75 



The- average number of observations from which any one of the ahove ^ix means 

 wric deduced, is over 800, and the probable error, in scale divisions, of any one of 

 the results is 4- 0.7 (nearly). 



From the Makerstoun observations, Broun found for the years 1843-46, a 

 minimum at the time of the full moon, and a maximum at the time of the new 

 moon; Kreil, from the Prague observations, between 1843-46, found the same 

 result, all in accordance with the Philadelphia results, as given above. It must be 

 remarked, however, that after the year 1848, Kreil found that the signs were 

 reversed and consequently it appears that the lunar influence on the horizontal 

 force is subject to a cycle of short period. This last remark does not apply to the 

 effect of the moon's declination and variations in distance. 



Influence of the Moon's Changes of Declination on th< Horizontal Force. — The 

 method of investigation is precisely the same as that adopted for the phases. We 



rind :— 



Table XII. 



One day before the greatest north declination 

 On the day of " " " " 



One day after " " " " 



Two days after " " " " 



On the day of the moon's crossing the equator 



One day before the greatest south declination 

 On the day of " " " " 



One day after " " " 



Two days after " " " 



Scale divisions 

 +0.8 



+0.6 

 +2.2 

 + 0.0 



—1.2 



—3.4 

 —0.9 

 +0.9 

 +1.0 



Mean +1.1. 



l'robable error of any one result ' 0.9. 



i 



Mean —0.6. 



It seems probable that the greatest effect takes place rather a day after than on 

 the day of the moon's greatest declination. Taking means, as indicated in the 

 above table, wc find about the time of the maximum north declination an increase 

 of horizontal force of 1.1 scale divisions (or 0.000040 parts of the horizontal force): 

 at the time of the moon's crossing the equator the force is decreased 1.2 scale divi- 

 sions (or 0.000044 parts of the horizontal force); the horizontal force also appears 

 decreased about the time of the moon's greatest south declination ; the amount is 

 about half that of the other two cases, and is somewhat doubtful, from an apparently 

 excessive value on the preceding day. 



