to the Antarctic Regions. 241 



triple observation, the positioft of the magnets being noted five mi- 

 nutes before and after the regular hour. The time of this triple ob- 

 servation will be 2 P.M., Gottingen mean time. 



The barometer, and the wet and dry thermometers, will be regis- 

 tered at each of the twelve magnetic hours. No observation will 

 be taken on Sunday. 



No distinct series of observations is required for the determination 

 of the secular variations. In the case of the declination, the yearly 

 change will be obtained by a comparison of the monthly mean re- 

 sults (for the same month and same hour) in successive years. The 

 observations of two years only will thus furnish 144 separate results, 

 from which both the periodical and the irregular changes are elimi- 

 nated ; so that great precision may be expected in the final result, 

 notwithstanding the limited period of observation. The same mode 

 of reduction will apply to the two components of the intensity^ pro- 

 vided that no change shall have taken place in the magnetic moment 

 of the bars employed. In the latter event, recourse must be had to 

 the absolute determinations for a knowledge of the secular changes. 



The subject of the irregular movements has acquired a prominent, 

 and almost absorbing interest, from the recent discoveries of Gauss. 

 Jt has been ascertained that the resultant direction of the forces, by 

 which the horizontal needle is actuated at a given place, is incessantly 

 varying, the oscillations being sometimes small, sometimes very con- 

 siderable ; — that similar fluctuations occur at the most distant parts 

 of the earth's surface, at which corresponding observations have been 

 as yet made ; — and that the instant of their occurrence is the same 

 everywhere. The intensity of the horizontal force has beea found 

 subject to analogous perturbations. 



For the full elucidation of the laws of these most interesting phe- 

 nomena, it is of the first importance that the stations of observation 

 should be separated as widely as possible over the earth's surface, 

 and that their positions should be chosen near the points of maxima 

 and minima of the magnetic elements. This has been in a great mea- 

 sure accomplished as regards the observatories about to be founded 

 by Her Majesty's Government. The stations are wide asunder in 

 geographical position, and they are in the neighbourhood of points 

 of prominent interest in reference to the isodynamic lines. The re- 

 sults of observation at these stations will soon testify whether the 

 shocks to wliich the magnetic needle is subject, are of a local or of 

 a universal character as regards the globe ; and in either event we 

 may expect that they will furnish information of great value (in re- 

 ference to a physical cause) as to the magnitude of the phenomena 

 in different places, and the elements on which it depends. 



In the observations destined to illustrate these phenomena, it is 

 proposed to follow, as nearly as possible, the plan laid down by 

 Gauss. One day in each month, namely, the last Saturday, will be 

 devoted to simultaneous observations on this system* ; the observa- 

 tions commencing at 10 p.m. of the preceding eve (Gottingen mean 

 time), and continuing through the 24 hours. 



* For the details of the arrangements of the " term observations,^' see the trans- 

 lation of Gauss's memoir in Tavlor's Scientific Memoirs, vol. ii. part v. 

 PhU. Ma^. S. 3, Vol, 15. No, 95. Sept. 1839. R 



