240 Instructions for the Scientific Expedition 



from one moment to another, and which observe (apparently) no 

 law. 



The periodical variations (with the exception of those of the decli- 

 nation) have hitherto been little studied ; and, even in the case of 

 the single element just mentioned, the results have scarcely gone 

 beyond a general indication of the hours of maxima and minima, 

 and of the changes of their amount with the season. The subject 

 is nevertheless of the highest importance in a theoretical point of 

 view. The phenomena depend, it is manifest, on the action of solar 

 heat, operating probably through the medium of thermoelectric cur- 

 rents induced on the earth's surface. Beyond this rude guess, 

 however, nothing is as yet known of the physical cause. It is even 

 still a matter of speculation whether the solar influence be a prin- 

 cipal, or only a subordinate cause, in the phenomena of terrestrial 

 magnetism. In the former case, the periodical changes are to be 

 regarded as the effect only of the variations of that influence ; in 

 the latter, they must be considered as its entire result, the action in 

 this case only serving to modify the phenomena due to some more 

 potent cause. It may be fairly hoped that a diligent study of this 

 class of phenomena will not only illustrate this and other doubtful 

 points in the physical foundation of the science ; but also, whenever 

 that physical cause shall come to be fully known, and be made the 

 basis of a mathematical theory, the results obtained will serve to give 

 to the latter a numerical expression, and to test its truth. Even 

 the knowledge of the empirical laws of the hourly and monthly fluc- 

 tuations must prove a considerable accession to science ; and (as one 

 of its more obvious applications) will enable the observer to reduce 

 his results, as far as this class of changes is concerned, to their 

 mean values. 



" For the complete determination of the hourly and monthly changes 

 of the magnetic elements, a persevering and laborious system of ob- 

 servation is requisite. The irregular changes are so frequent, and 

 often so considerable, as (partially at least) to mask the regular ; 

 and the observations must be long continued at the same hours, be- 

 fore we can be assured that the irregularities do not sensibly aflfect 

 the mean results. Again, in a theoretical point of view, the noc- 

 turnal branch of the curves by which the periodical changes are re- 

 presented is quite as important as the diurnal ; and it is manifest 

 that nothing can be done towards its determination without the co- 

 operation of a number of observers. At each of the observatories 

 about to be founded by the liberality of Her Majesty's Government, 

 there will be three assistant observers placed under the command of 

 the director ; and it is intended that the observations shall be taken 

 every two hours throughout the twenty-four. In order that this 

 series of observations, which is especially destined for the determi- 

 nation of the periodical changes, may at the same time cast some 

 light upon the irregular movements, it is proposed that they shall 

 be simultaneous at all the observatories. The hours which have been 

 agreed upon are the even hours (0, 2, 4, 6, &c.) Gottingen mean time. 

 It is likewise intended that one observation of the twelve shall be a 



