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



The preceding facts and queries were addressed to you two 

 months before this date ; the letter was then shown to different 

 persons, and among others to Lieut.-General Cullen the British 

 resident here, with whom you are no doubt acquainted as a 

 zealous inquirer in geology and terrestrial physics for the last 

 half century in India. I had also communicated my results and 

 the greater part of my guesses to that gentleman before. 



I am, my dear Sir, 



Yours very truly, 



John Allan Broun. 



P.S, — Since this letter was written I have seen in the Bihlio- 

 theque Universelle de Geneve, July 1857, a translation of a me- 

 moir by M. Secchi on the periodic variations of terrestrial mag- 

 netism*. This is a continuation of a former paper translated in 

 the Philosophical Magazine for November 1854 and June 1855, 

 and it draws from me a correction and a few remarks. 



In his second paper, M. Secchi offers some laws of the move- 

 ments of a needle freely suspended in the direction of dip. 

 Tliese movements were obtained by me first, I believe, from all 

 the three magnetic instruments, and exhibited to the British 

 Association in 1846; the curves produced by the movement of 

 the north end of the needle were given for each month of the 

 year, in the volume of Makerstoun Observations which M. Sec- 

 chi has consulted; yet he says, "Mr. Broun has given analogous 

 curves for Makerstoun, and a glance at these will show the same 

 law, though somewhat more complicated from having grouped 

 too many months together, and from the higher latitude and fre- 

 quent disturbances." M. Secchi has examined plate 8 instead of 

 plate 7, which contains the cui-ves for each month (Trans. Koyal 

 Soc. Edinb. vol. xix. p. ii.). In plate 8 the figures are given for 

 periods of 60 and 90 instead of 30 days ; but this combination 

 can scarcely be said to complicate the curves, as an examination 

 of plate 7 will show. In plate 8, however, I have also pro- 

 jected the curves obtained from widisturbed movements, and 

 have thus shown the effect of disturbance in displacing the 

 curves. Various conclusions deduced from these figures are 

 given in pages Ixx-lxxiv; they are the only ones of the kind that 

 have been projected, as far as I know, till ]\I. Secchi has now 

 constructed one for the whole year {Bihliotheque de Geneve, 

 p. 164). This curve, of course, may be considered complicated, 



ure ; and I have noted a resemblance between the annual law of difference 

 of atmospheric pressure atMakerstoun and Greenwich, and the annual law 

 of horizontal magnetic force. Do not the differences of the atmospheric 

 pressures at the same two places, from year to year, show some resemblance 

 to the decennial law, 1843-44 being an epoch of minimum ? (Trans. Roy. 

 Soc. Edinb. vol. xix. part 2. p. xci.) 



