392 M, A. Seech i on the Periodical Variations 



that by taking a good clue as guide, it will be found here also 

 that all is number and measure. 



From the rapid sketch which I have traced of what has been 

 already done for the study of the variations of terrestrial mag- 

 netism, it will be clear that observations are not wanting ; and 

 what is more, that they have been made systematically and with 

 extraordinary perseverance, guided by experience, and with unex- 

 ceptionable instruments ; whence we may conclude, that if there 

 were obtained at any place results opposed to the laws deduced 

 from so imposing a mass of data, amounting to several millions, 

 they ought to be attributed to faults either in the instruments, 

 the observations, or the methods of reduction ; in short, to some 

 local or accidental cause ; but we shall see that the results are 

 more accordant than would be believed, and it is of these that 

 we now proceed to speak. 



Part II. — Principal results obtained from the study of the 

 Magnetic Variations. 



Magnetic variations are of three kinds; the first ordinary 

 and periodical ; the second extraordinary, and apparently irre- 

 gular ; and the third requiring for their completion a great and 

 unknown series of years, and therefore termed secular. All the 

 magnetic elements, i. e. the Declination, the Intensity of the Force, 

 and the Inclination, are subject to these variations. We will 

 begin with what belongs to the declination and its movements, 

 which fall under some principal laws, which we will proceed to 

 expose. 



§ I . On the Diurnal and Annual Variations of the Declination, 



First Law. — " The diurnal variations of the magnetic needle 

 follow local time.'' 



Declaration. — The first discoverers of the diurnal variations of 

 the magnetic declination suspected that the needle followed the 

 course of the sun, and therefore the true (or apparent) time of 

 the place of observation ; but when it was afterwards found, by 

 comparative observations, that there were cotemporaneous varia- 

 tions at many different places, it was suspected that there might 

 be simultaneity of perturbation throughout the globe. When 

 however places of observation sufficiently distant were multiplied, 

 it was found that the ordinaiy, or diurnal variations, followed in 

 their march the hours of local time, and that even the extra- 

 ordinary variations, as we shall see in the appropriate place, were 

 not completely excluded from the operation of this law. To 

 avoid speaking equivocally, however, the term " distance" must 

 be understood in relation to the subject of which we are treating. 

 The extent even of the whole of Europe, and still more distances 



