Mr. J. A. Broun on Terrestrial Magnetism. 93 



examination of the observations at Munich and Toronto. An- 

 other period obtained by General Sabine I showed was due to 

 error in the temperature coefficient (Trans. Roy. Soc. Edinb. 

 vol. xvi. p. 102). 



To this result I have now to add, that the relative changes of 

 mean hoi'izontal force are the same over the globe, and the 

 changes from day to day of the mean horizontal force at differ- 

 ent places on the earth's surface are nearly equal, the unit in 

 each case being the whole value of the horizontal force at the 

 place. In other words, the changes of mean horizontal foi-ce, 

 from day to day, are in the same direction over the globe, and 

 are proportional to the horizontal forces at the places; the diffei- 

 ent effect of disturbance, due to its diurnal period, and the 

 different directions of the secular change, being allowed for. 



The preceding and other facts, which I shall shortly endea- 

 vour to prove in detailed memoirs, have led me to consider anew 

 the different hypotheses offered to explain the magnetic varia- 

 tions. I had already satisfied myself, from the Makerstoun 

 Observations, that the theories depending on temperature were 

 wholly insufficient. I showed that the areas of the declination- 

 cuiTes (lid not follow the law of temperature, as supposed by 

 some physicists. I also showed that such areas were not the 

 proper elements in comparison for such a purpose. If any areas 

 were employed, it should be the areas of the curve described by 

 the needle freely suspended in the direction of the magnetic force. 



The results I have now given prove, it appears to me, the 

 insufficiency of any theory that attempts to relate the amount of 

 the diurnal movement to the temperature of the place. March 

 is with us here nearly the hottest month of the year, whereas 

 August is about the coolest ; yet the former is the month of 

 the minimum, and the latter the month of maximum diurnal 

 movement of the declination-magnet. 



None of the facts previously known seem to have been suffi- 

 ciently conclusive ou this point, since Dr. Faraday has proposed 

 a new theory which depends on the temperature of the sun, 

 acting on our magnetic atmosphere. There can be no difference 

 of opinion as to the great ingenuity displayed by Dr. Faraday 

 in his attempts to make his theory explain the results of obser- . 

 vations. No doubt many of his applications are not very clear 

 to me; thus he explains the different motions of the dipping- 

 needle at St. Helena and llobarton by the statement, "that as 

 the region is located above in the air, it is above the angle which 

 the dip makes with the horizon at St. Helena, and therefore ought 

 to depress the line of force and lessen the dip. At Hobarton, 

 the region being in the tropical jjarts, is within the angle formed 

 by the line of dip with the horizon, and therefore deflects the 



