Mr. J. A. Broun on Terrestrial Magnetism. 83 



needle were mosf irregular in the evening was remarked by Cas- 

 sini (and probably by earlier observers) ; but, as far as I am 

 aware, the first careful investigation is due to M. Kreil. This 

 physicist concluded that the perturbations were a maximum in 

 the evening from S^ to 10^ p.m., and a minimum in the morning 

 from 8^ to 10'' a.m. The law for the disturbance of force was 

 found to have different epochs. These conclusions were verified 

 by difi'erent persons, and by myself to a certain extent. 



I found that these results were on the whole due to a combi- 

 nation of different and even opposite laws ; my conclusion was as 

 follows : — 



" It appears from these results that the diurnal law of mean 

 disturbance is not constant throughout the year, as has been sup- 

 posed ; in fact, the law for the summer is nearly the reverse of 

 that for winter, while that for autumn is nearly intermediate 

 between the two" (Trans. Roy. Soc. Edinb. vol. xix. part 2. 

 p. xxviii). 



In 1856 I communicated the fact to Lieut. -Colonel Sj'kes and 

 others, that the law of disturbance near the equator was the in- 

 verse of the so-called mean law for Europe ; that is to say, near 

 the equator the diurnal law of disturbance throughout the year 

 resembles on the whole the law for Europe in summer, the maxi- 

 mum occurring about 10 a.m. and the minimum about 10 p.m. 

 I have therefore not only given the diurnal law of di!«turbance 

 for each season in Europe, but I have also found that this law 

 varies both with season and latitude. These results refer to the 

 mean disturbance without reference to sign. When the sign or 

 direction of movement was considered, it was found that the 

 maximum positive disturbance coincided on the whole with the 

 minimum negative disturbance. This is not the case near the 

 equator; the law for both signs is the same. It is probable, 

 therefore, that for intermediate latitudes the relation of the law 

 of signs will vary. We may expect in some intermediate latitude 

 that the mean disturbance of declination will be equal at 10 a.m. 

 and 10 P.M., and that the hour of maximum disturbance of force 

 will occur gradually later after noon as we leave the equator and 

 approach the poles. 



Monthly laws of disturbance. — I found that the mean depart- 

 ure of the declination-needle from its monthly mean positions 

 at the corresponding hours, was greatest two or three days after 

 the moon's opposition, and was least about the time of conjunc- 

 tion (Trans. Uoy. Soc. Edinb. vol. xix. part 2. p. x\). 1 also 

 concluded that the diurnal range of the deelination-ncedle was 

 greatest when the moon was near the equator (Ibid. p. xix) : 

 similar laws were found for the components of force. 



Annual lavs of disturbance. — The mean departure of the de- 

 G 2 



