84 jMr. J. A. Broun on Terrestrial Magnetism. 



clination -needle from the normal position for the corresponding 

 hour was found by rae to be greatest near the equinoxes, and 

 least at the summer and winter solstices (Ibid. p. xxii). This 

 law was published by me in 1846. At that time some investi- 

 gations had been made to determine the law. M. Kreil and 

 General Sabine had found the epoch of maximum disturbance to 

 be in winter, and Dr. Lloyd had ascertained it to be in summer. 

 General Sabine's result, it is true, was derived from disturbance 

 above a certain arbitrary limit, — a fact, however, which I showed 

 could not have affected the general conclusion. Since then 

 General Sabine has verified on the whole my result by his own 

 process. 



I may add here, to complete these laws, that first indicated by 

 Dr. Lamont, namely that the diurnal movement seems to have 

 been a maximum in 1838 and 1848, and a minimum in 1843-44, 

 or that the amount of the magnetic variation seems to obey a 

 decennial law ; a result which General Sabine has also found to 

 exist in bis discussion for the value of disturbance. 



It may be seeu from the results of the Makerstoun Observa- 

 tions (Trans. Hoy. Soc. Ediub. vol. xix. part 2. pp. xii, xxxi, xlv), 

 that the secular change seems on the whole to obey a similar 

 law. This fact I noticed at the time (Ibid. No. 5. p. xii.) ; but 

 as the changes were probably in part due to instrumental causes, 

 especially in the first years of the series, I could not offer any 

 general conclusion. An examination by me of observations 

 made in other places, however, seems to confirm the results ob- 

 tained at Makerstoun, which may be stated thus : — 



The secular rate of increase of horizontal force was a maximum 

 in 1843-44, and a minimum in 1847-48. 



The secular rate of diriiinution of the vertical force was a mi- 

 nimum in 1848. As 1842 was the first year for which a result 

 has been obtained, the epoch of maximum is uncertain. 



The rate of yearly change of magnetic declination seemed to 

 attain a maximinii in 1847-48. 



As already mentioned, the results in the first years (1 842-44) 

 were probably affected by instrumental errors ; but this could 

 not be the case in the latter years ; and this connexion with the 

 decennial period for three instruments cannot be accidental. 



These results were evidently connected with the increasing 

 amount of disturbance ; and at first sight it might appear that 

 the variation of the secular change was due to disturbance : thus 

 the disturbance tends to diminish the value of the horizontal 

 force, the secular change being a yearly increase of force ; the 

 greater the amount of disturbance, the less the yearly increase 

 should appear. This relation, however, does not appear to apply 

 either to the vertical force or to the declination ; and I am in- 



