of Terrestrial Magnetism* ' 393^ 



of six or seven hundred miles, are very small compared to the 

 entire circumference of the globe. In the same manner that 

 many meteorological vicissitudes may be simultaneous for such 

 extents, so may also the magnetic perturbations which might be 

 produced by them ; but as it can rarely happen that meteoro- 

 logical causes occupy the whole surface of the earth, so simul- 

 taneous perturbations produced by them and extending over the 

 whole globe would be equally rare. 



In fact, if we inspect the magnetic curves, traced in Gottingen 

 time, for Gottingen and Prague in Europe, and for places situated 

 in Canada and the United States of America, we shall find that 

 the places in each continent commonly agree very well with each 

 other; but that agreement between the continents is seldom 

 found, although their distance apart is not great compared to 

 the whole globe. 



It is necessary however to discriminate accurately between two 

 kinds of periodical variations ; those which strictly follow local 

 time, and those which in their periods occur at the same moment 

 of absolute time at different stations. We shall speak of the 

 latter subsequently; but in respect to the former, let it be 

 regarded as assured that local time is to be alone considered ; 

 and that if Gottingen time was at first adopted for all the ob^ 

 servatories in common, it was for the sake of making out the 

 law of the extraordinary movements and facilitating their calcu- 

 lation, rather than for the purpose of recognizing the law of the 

 diurnal variations of which we are now speaking. 



It would indeed have been desirable to have adopted true or 

 apparent local solar time, instead of mean time, in the observa- 

 tions, or at least in the reductions. The use of mean Gottingen 

 time, besides the inconvenience of requiring the equation of time^ 

 to be applied, has also another, which is, that it does not often ; 

 happen that at two distant observatories the observations fall at 

 even hours of local time. This is one of the points (and we 

 shall see others presently) in which the discussion of past obser- 

 vations throws light on the system to be adopted in future. It is 

 to be hoped that future observations will be made at even hours- 

 of apparent local time, and that those which have been made 

 will be reduced to such hours. 



We have however found, and shall demonstrate in the sequel, 

 that the phase of the diurnal oscillations depends more on the 

 position of the sun relatively to the magnetic meridian of a given 

 place (i. e. relatively to the azimuth of the plane of the mag- 

 netic meridian) than on the relation to the geographic meridian. 

 Second Law. — " The pole of the needle which is least distant 

 from the sun makes a double diurnal excursion, in the following 

 manner : — It is at its maximum of western excursion four or five 

 Phil. Mag, S. 4. Vol. 8. No. 53. Nov, 1854. 2 D 



