AL PHILOSOPHY. 321 



a view of magnetic action so singular, and so utterly unexpected, as to 

 amount to the creation of a new department of science, and a detection of a 

 completely novel system of physical relations ; for that, in the first place, 

 the systems of diurnal and annual magnetic changes have each been sepa- 

 rated into two perfectly distinct and physically independent systems, the 

 one, at any particular station, holding its course according to laws depend- 

 ing solely on the sun's hour-angle at the moment of observation, and his 

 meridian altitude at different seasons, the other, comprehending all those 

 movements which, under the name of magnetic storms, or " irregular dis- 

 turbances," have hitherto presented the perplexing aspect of phenomena 

 purely casual, capricious in amount and in the particular occasions of their 

 occurrence when regarded singly, has been shown, by these discussions, to 

 be subject in its totality to laws equally definite with the others, though 

 more dependent for their application on peculiarities of local situation. As 

 regards the first of these fluctuations, they find it demonstrated: 



That the sun's regular action on the magnetism of the globe is determined 

 by a law of no small complexity and intricacy, but which, nevertheless, has 

 been traced with precision and certainty, and shown to be referable, in the 

 first place, and for one of its arbitrary coefficients, to the geographical situ- 

 ation of the place of observation with respect to a certain line or equator on 

 the earth's surface, which cannot yet be precisely traced for want of suffi- 

 ciently numerous stations (but which seems to approach to the line of least 

 intensity, and is very far from coinciding with the geographical equator), - 

 and in the next, and for its other influential cause, to the fact of the sun's 

 having north or south declination; so that the wnole diurnal change in any 

 one of the elements, and at any station, is made up of two portions, one of 

 which retains the same sign and a constant coefficient all the year round; 

 the other changes sign, and varies in the value of its coefficient with the 

 annual movement of the sun from one side of the equator to the other. 



That, consequently, for a station on the magnetic equator (so defined), the 

 mean amount of diurnal change is nil, when taken over the whole year, but 

 that on any particular day of the year it has a determinate magnitude, which 

 passes through an annual periodicity, with opposite characters in opposite 

 seasons. And that for a station in middle latitudes the mean diurnal fluctu- 

 ation is not nil, but such as, during every part of the year, to exhibit an east- 

 erly deviation in the morning hours, and a westerly in the evening hours, 

 for stations north of the equator, and vice versa for those south of it ; but that 

 the amount of this deviation, or the amplitude of the diurnal fluctuation, 

 varies with the seasons, being exaggerated or partially counteracted by the 

 alternate conspiring and opposing influence of the sun's declination during 

 the summer and winter seasons. 



As regards the irregular disturbances, though arbitrary and capricious in 

 extent, and in the moments when they may be expected, individually, this 

 does not pi-event their obeying, with great fidelity, the law of averages, when 

 grouped in masses, and treated separately from those of the former class. 

 So handled they are found to conform, in their average effect, at each of the 

 twenty-four hours of the day, and on each day of the year, to the very same 

 rules, as regards the sun's daily and annual movement, with one remarkable 

 point of difference, viz., that their hours of maxima and minima are not 

 identical with those of the regular class, but that each particular station has, 

 in this respect, its own peculiar hours, analogous to what is called the "estab- 

 lishment " of a port in the theory of the tides. And that, in consequence, the 



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