to the Antarctic Regions. 185 



^t> 



are subject to similar perturbations. Recently, at many of the 

 German stations, the horizontal component of the intensity has been 

 observed, as well as the declination ; but the determination of an- 

 other element is yet required, before we are possessed of all the data 

 necessary in this most interesting research. 



The magnetic observations about to be established in the British 

 Colonies, by the liberality of the Government, will (it is hoped) 

 supply in a great measure these desiderata. The stations are widely 

 scattered over the earth's surface, and are situated at points of pro- 

 minent interest with regard to the Isodynamic and Isoclinal lines. 

 The point of maximum intensity in the northern hemisphere is in 

 Canada; the corresponding maximum in the southern hemisphere is 

 near Van Diemen's Land ; St. Helena is close to the line of minimum 

 intensity ; and the Cape of Good Hope is of importance on account 

 of its southern latitude. At each observatory the changes of the ver- 

 tical component of the magnetic force will be observed, as well as those 

 of the horizontal component and declination ; and the variations of the 

 two components of the force being known, those of the inclination 

 and of the force itself are readily deduced. The simultaneous obser- 

 vations of these three elements will be made at numerous and stated 

 periods, and we have every reason to hope that the directors of the 

 various European obseiTatories will take part in the combined system. 



But interesting as these phenomena are, they form but a small 

 part of the proper business of an observatory. The regular changes 

 (both periodic and secular) are no less important than the irregular ; 

 and they are certainly those by which a patient inductive inquirer 

 would seek to ascend to general laws. Even the empirical expres- 

 sion of these laws cannot fail to be of the utmost value, as furnish- 

 ing a correction to the absolute values of the magnetic elements, and 

 thereby reducing tliem to their mean amount. 



The hourly changes of the declination have been frequently and 

 attentively observed ; but with respect to the periodical variations 

 of the other two elements, our information is as yet very scanty. The 

 determination of these variations will form an important part of the 

 duty of the magnetic observatories ; and from the accuracy of which 

 the observations are suscej itible, and the extent which it is proposed 

 to give them, there can be no doubt that a very exact knowledge of 

 the empirical laws will be the result. 



With respect to the secular variations, it might perhaps be doubted 

 whether the limited time during which the observatories will be in 

 operation is adequate to their determination. But it should be kept 

 in mind that the monthly mean corresponding to each hour of ob- 

 servation will furnish a separate result ; and that the number and 

 accuracy of the results thus obtained may be such, as fully to com- 

 pensate for the shortness of the interval through which they are fol- 

 lowed. A beautiful example of such a result, deduced from three 

 years' observation of the declination, is to be found in the first vo- 

 lume of Gauss's magnetical work, of which a translation is published 

 in the fifth number of Taylor's Scientific Memoirs. 



