Mr. J. A. Broun on Terrestrial Magnetism. 99 



since the figure deduced from the mean of 365 days cannot be 

 said to resemble that for any day of the year. 



M. Secchi deduces from his curve the following conclu- 

 sions : — 



"Second Law. — The disturbed curve is the ordinary curve 

 entirely displaced by a certain quantity.'' A glance' at the 

 Makerstoun curves (plate 8) will show that this conclusion is 

 inexact. Doubtless there is a displacement, and, as the efiect of 

 distm-bance is small compared with the whole movement, the 

 curve cannot be greatly changed in form ; but the change of 

 figure is quite as great as the amount of perturbation admits. 

 M. Secchi also finds — 



" Third Law. — By the effect of the disturbances, the curve tends 

 always to become more symmetrical and equal in its two lobes." 



Again, it appears to me that a glance at plate 8 will show that 

 this conclusion is not wan-anted. It seems to me, on the con- 

 trarj', that the effect of the disturbance is to render the figures 

 less symmetrical, the curves for March and April perhaps ex- 

 cepted. The tendency of the disturbance is to introduce a loop 

 in the easterly or night movement ; and this is so evident that it 

 seems probable, if we could obtain curves perfectly free from dis- 

 turbance, the loop to the east in the night would not exist. 



M. Secchi adds, " These conclusions, deduced from the obser- 

 vations of the three magnetic elements at Toronto, will be verified 

 without doubt in other countries for which the discussion has 

 not yet been performed." I refer to the Makerstoun Observa- 

 tions, where the discussion was published eight years ago, and 

 where all the details will be found. "With reference to his fourth 

 law, given in a former part of this letter, namely, that ''the dis- 

 turbances are a maximum at the equinoxes, and a minimum at 

 the solstices," M. Secchi remarks, "2nd. At the solstices, the 

 poles of the sun are turned towards the earth, so that its mag- 

 netic action should be more powerful." Solstices must have 

 been written by mistake for equinoxes, since it is near the latter 

 periods that the poles of the sun are turned towards the earth ; 

 but the conclusion is a statement of the result obtained by me, 

 and referred to j)reviously under the head of "Annual law of 

 Force," on the supposition that the north pole of tlie sun con- 

 tains the same magnetism as the north pole of our earth. — J. A. B. 



2nd P.S. I should have Pitntioncd a fact in connexion with 

 the historyof the secular movements that I have not yet published. 

 The magnetic declination is nearly half a degree cast at Trcvan- 

 drum. Before the year ISSt, the movement was towards the 

 north, that i.s, westerly. I have found that it began to turn east- 

 erly in the year 18.j 1, and that the rate of motion yearly towards 

 the east is becoming greater and greater. — J. A. i3. 



112 



