178 Instructions for the Scientific Expedition 



tions. The first comprehends the actual distribution of the magnetic 

 influence over the globe, at the present epoch, in its mean or average 

 state, when the effects of temporary fluctuations are either neglected 

 or eliminated by extending the observations over a sufficient time to 

 neutralize their eff^ects. The other comprises the history of all that 

 is not permanent in the phenomena, whether it appear in the form 

 of momentary, daily, monthly, or annual change and restoration, or 

 in progressive changes not compensated by counter changes, but 

 going on continually accumulating in one direction, so as in the 

 course of many years to alter the mean amount of the quantities ob- 

 served. These last-mentioned changes hold the same place. In the 

 analogy above alluded to, with respect to the mean quantities and 

 temjDorary fluctuations, that the secular variations in the planetary 

 movements must be regarded as holding, with respect to their mean 

 orbits on the one hand, and their perturbations of brief period on the 

 other. 



There is, however, this dlfi^erence, that in the planetary theory all 

 these varieties of eff'ect have been satisfactorily traced up to a single 

 cause, whereas in that of terrestrial magnetism this is so far from 

 being demonstrably the case, that the contrary is not destitute of 

 considerable probability. In fact, the great features of the magnetic 

 curves, and their general displacements and changes of form over the 

 whole surface of the earth, would seem to be the result of causes 

 acting in the interior of the earth, and pervading its whole mass; 

 while the annual and diurnal variations of the needle, with their 

 train of subordinate periodical movements, may, and very probably 

 do arise from, and correspond to electric currents produced by pe- 

 riodical variations of temperature at its surface, due to' the sun's po- 

 sition above the horizon, or in the ecliptic, modified by local causes ; 

 while local or temporary electric discharges, due to thermic, che- 

 mical, or mechanical causes, acting in the higher regions of the at- 

 mosphere, and relieving themselves irregularly or at intervals, may 

 serve to render account of those unceasing, and as they seem to us 

 casual movements, which recent observations have placed In so con- 

 spicuous and interesting a light. The electrodynamic theory, which 

 refers all magnetism to electric currents, is silent as to the causes of 

 those currents, which may be various, and which only the analysis 

 of their effects can teach us to regard as internal, superficial, or at- 

 mospheric. 



It is not merely for the use of the navigator that charts, giving 

 a general view of the lines of Magnetic Declination, Inclination, 

 and Intensity, are necessary. Such charts, could they really be 

 depended on, and were they in any degree complete, would be of 

 the most eminent use to the theoretical inquirer, not only as ge- 

 neral directions in the choice of empirical formulae, but as powerful 

 instruments for facilitating numerical investigation, by the choice 

 they afford of data favourably arranged ; and above all, as affording 

 decidedly the best means of comparing any given theory with ob- 

 servation. In fact, upon the whole, the readiest, and beyond com- 

 parison the fairest and most eflfectual mode of testing the numerical 

 applicability of a theory of terrestrial magnetism, would be, not ser- 



