NOTES TO BOOK XII. 75 



I can only allude to Gauss's investigations respecting 

 the Absolute Measure of the Earth's Magnetic Force. To 

 determine the ratio of the magnetic force of the earth to 

 that of a known magnet, Poisson proposed to observe the 

 time of vibration of a second magnet. The method of 

 Gauss, now universally adopted, consists in observing the 

 position of equilibrium of the second magnet when de- 

 flected by the first. 



The manner in which the business of magnetic obser- 

 vation has been taken up by the governments of our time 

 makes this by far the greatest scientific undertaking which 

 the world has ever seen. The result will be that we 

 shall, obtain in a few years a knowledge of the magnetic 

 constitution of the earth which otherwise it might have 

 required centuries to accumulate. The secular magnetic 

 changes must still require a long time to reduce to their 

 laws of phenomena, except observation be anticipated or 

 assisted by some happy discovery as to the causes of these 

 changes. But besides the special gain to magnetic science 

 by' this great plan of joint action among the nations of 

 the earth, there is thereby a beginning made in the recog- 

 nition and execution of the duty of forwarding science in 

 general by national exertions. For at most of the mag- 

 netic observations, meterological observations are also car- 

 ried on ; and such observations, being far more extensive, 

 systematic, and permanent than those which have usually 

 been made, can hardly fail to produce important additions 

 to science. But at any rate they do for science that 

 which nations can do, and individuals cannot ; and they 

 seek for scientific truths in a manner suitable to the 

 respect now professed for science and to the. progress 

 which its methods have made. Nor are we to overlook 



