M. J. A. Broun on the Variation of Magnetic Declination* 69 



There is a lunar diurnal law of disturbance which, from the mean 

 of six years' observation, consists of a 



Principal maximum 5 hours before the moon passes the upper 

 meridian. 



Principal minimum near the moon's passage of the lower meridian. 



Secondary maximum 6 hours after the passage of the upper meri- 

 dian. 



Secondary minimum 1 hour after the passage of the upper meri- 

 dian. 



"When the results for each year are compared, the principal maxi- 

 mum and minimum are always shown distinctly; but the values and 

 epochs of the secondary maximum and minimum vary, and that so 

 irregularly, that in the mean for the three years 1857-59 the exist- 

 ence of either is doubtful, only the principal maximum and minimum 

 being well-marked. 



In order to determine if the law varied with season, means of groups 

 for quarters of the year were obtained ; means also for the groups of 

 half-years, October to March and April to September, were taken. As 

 the results for the quarters forming each half-year agreed with each 

 other, and that for the half-year which contained them, the following 

 conclusions may be accepted as probably true. 



Half-year October to March, 



Principal maximum, 5| hours before the moon's passage of the 

 upper meridian. 



Principal minimum, 3 hours before the moon's passage of the lower 

 meridian. 



Secondary maximum, 5| hours after the moon's passage of the 

 upper meridian. 



Secondary minimum, 1 hour after the moon's passage of the upper 

 meridian. 



Half-year April to September. 



Maximum, A\ hours before the moon's passageof the upper meridian. 

 Minimum, at the passage of the lower meridian. 



In the case of the half-year including the June solstice, the secon- 

 dary points disappear ; this also is the case for each of the quarters 

 forming the half-year. It may be due to this and the preponderance 

 of the disturbance in one half of any given year, that the mean for 

 the whole year shows distinctly or indistinctly the secondary points . 

 the question, however, of the secondary points will be considered here* 

 after in another discussion including a longer series of observations. 



The mean value of the lunar disturbance of magnetic) _ ^f.ojt 

 declination for the six years 1854-59 at Trevandrum . . ) 



The maximum value at 5 hours before the upper passage = 0'*3G3 

 The minimum value near the inferior passage = 0'*324 



So that the variation of the mean law is only 0'*04, or about two 

 seconds and a half. 



