Royal Society. 141 



magnetism of the earth Itself. The geographical aspect, if we may 

 so express it, of the terrestrial magnetism, or the different measure 

 in which the magnetic force exists at different parts of the earth's 

 surface, and the different directions which a magnet assumes in dif- 

 ferent places by virtue of this force, so far from being permanent, 

 are found to be subject to a continual change, which differs from all 

 other magnetic variations with which we are acquainted, inasmuch 

 as it does not present to us the character of an oscillation of the 

 phaenomena around a mean value in periods of greater or less dura- 

 tion, but appears, especially when viewed generally in its operation 

 over the whole globe, as a continuously progressive change ; it has 

 for this reason received the appropriate name of * secular change.' 

 It is possible indeed that the magnetism of the earth may have 

 its periods, — that the phsenomena existing at one and the same epoch 

 over the whole surface of the globe may be identically reproduced at 

 a subsequent epoch, — and that what has been called the secular 

 change of each of the magnetic elements, which we perceive to be in 

 progress at any particular point of the surface, St. Helena for ex- 

 ample, may be part of a succession of changes which operate in a 

 cycle, of which the duration, vast as it may be, may hereafter be 

 found to be calculable. But as far as our knowledge has yet gone, 

 it is insufficient to justify the assumption of even approximate pe- 

 riodical laws of this variation of the terrestrial magnetism ; and 

 we must continue to regard it therefore for the present as a secular 

 change, of which the period, if there be one, or the periods, if there 

 be more than one, are as yet unknown. But although the secular 

 change has no intrinsic relation, as far as we have been able to dis- 

 cover, to any of the periods of time determined by other phsenomena, 

 either of our own planet or of any other of the heavenly bodies, it is 

 obvious that we may assign the average rate at which the change is 

 taking place, in any of the magnetic elements and at any particular 

 station (the declination for example at St. Helena), corresponding to 

 any definite measure of time in usage amongst us (say for example 

 a month, or the twelfth part of a solar year), by taking the successive 

 differences between the monthly means of all the hourly observations 

 in the first and second months of their continuance, then between 

 the second and third months, then between the third and fourth, and 

 so on. By thus proceeding in the case of the Declination at St. He- 

 lena, we have sixty differences thus accruing in the five years of 

 hourly observation, by which we find that the monthly increase of 

 West Declination during these five years amounted on the average 

 to 0'-657, or to an annual increase of 7'" 88. 



" It is not however necessary for this investigation that the system 

 of observation should be hourly : a much less onerous system is suf- 

 ficient, provided that the observations be distributed equably through 

 the year, and that the intervals between the observations of each 

 day be, approximately at least, equidistant. Before the commence- 

 ment of the hourly series there had been fifteen months of two- 

 hourly observations, and after its close the observations were con- 

 tinued for twenty-one months more at five hours of each day, the 

 hours being such as to give by their combination a true mean value 



