88 Mr. J. A. Broun on Terrestrial Magnetism. 



I shall now offer you some remarks on the diurnal variation 

 of the magnetic declination. 



In a paper on this subject, read before the Royal Society of 

 Edinburgh, May 3, 1847, while showing that the movement of 

 the needle relative to midnight was of the same character as that 

 relative to noon, I suggested that the apparent smallness of the 

 night maximum might be due to the variation being a resultant 

 of two opposite motions ; one, the greatest, having its maximum 

 near noon and its minimum near midnight ; the other, the 

 smaller, having its maximum near midnight and its minimum 

 near noon. This supposition I illustrated by the superposition 

 of two curves. 



I have also shown in the Makerstoun Observations for 1844 

 (Trans. Roy. Soc. Ediub. vol. xviii. p. 354), that if we subtract 

 the ordinates of the curve representing the diurnal movement of 

 the needle for the northern hemisphere (employing the curve for 

 summer at Makerstoun for this purpose) from the curve for the 

 winter at the same place, the differences will be the ordinates of 

 a curve representing the movement of the needle in the southern 

 hemisphere. 



The conclusion I drew from this fact was, that the reversal 

 of movement with the sun north and south of the equator ob- 

 served between the tropics was also shown in high latitudes ; and 

 that the midwinter curve at Makerstoun might be considered 

 due to the superposition of the two opposite curves representing 

 the movements in high north and south latitudes. M. Secchi 

 has since then made a similar proposition with the same object, 

 employing preferably the mean of the year as the typical move- 

 ment (Monthly Notices, Roy, Ast. Soc. vol. xv. p. 27). I have 

 little doubt still that the hypothesis of superposed opposed move- 

 ments may be employed to obtain all the variations observed in 

 different places at the same time, and in the same place at differ- 

 ent times. 



When M. Arago wrote his conclusion that there should be a 

 line of no diurnal range for the declination-needle, the following 

 were the facts in his possession. In the northern hemisphere, 

 the north end of a needle moves towards the west from the morn- 

 ing till afternoon, and towards the east after noon till the even- 

 ing, with a secondary oscillation at night. 



In the southern hemisphere, if we consider the north end of 

 the needle, its movements are in the opposite directions for the 

 same periods of the day. ; 



show a maximum in the number of meteors on the Sth, 10th, and 11th of 

 August for the year 1848. The number increases from 184.5, the time of 

 commencement of his observations, and diminishes till 1854. — Ann, de 

 Chim. January 1855, p. 4G. 



