NOTES. XIX 



1806) indicate the date of the observations; the figures within brackets, which 

 are added occasionally after the titles of the memoirs, show the date of the pub- 

 lication of the observations, which was often much later. 



(^ p. 69. Malus (1808) and Arago (1811 , chromatic) polarisation of light. 

 SeeKosmos, Bd. ii. S. 370 (English edition, p. 329). 



( 70 ) p. 70. Kosmos, Bd. i. S. 186 and 429, Anm. 17 (English edition, 

 p. 169 and Note 147). 



( 71 ) p. 72. " Before the practice was adopted of determining absolute values, 

 the most generally used scale (and which still continues to be very frequently 

 referred to) was founded on the time of vibration observed by Mr. de Humboldt 

 about the commencement of the present century, at a station in the Andes of 

 South America, where the direction of the dipping-needle was horizontal a con- 

 dition which was for some time erroneously supposed to be an indication of the 

 minimum of magnetic force at the Earth's surface. From a comparison of the 

 times of vibration of Mr. de Humboldt's needle in South America and in Paris, 

 the ratio of the magnetic force at Paris to what was supposed to be its minimum 

 was inferred (1'348); and from the results so obtained, combined with a similar 

 comparison made by myself between Paris and London in 1827 with several 

 magnets, the ratio of the force in London to that of Mr. de Humboldt's original 

 station in South America has been inferred to be 1'372 to I'OOO. This is the 

 origin of the number 1'372, which has been generally employed by British ob- 

 servers. By absolute measurements we are not only enabled to compare numeri- 

 cally with one another the results of experiments made in the most distant parts 

 of the globe, with apparatus not previously compared, but we also furnish the 

 means of comparing hereafter the intensity which exists at the present epoch 

 with that which may be found at future periods." Sabine, in the Manual for 

 the Use of the British Navy, 1849, p. 17. 



( 72 ) p. 74. The desirability of concerted simultaneous magnetic observation 

 was first perceived by Celsius. Without speaking of the influence of the aurora 

 upon the declination, which had been discovered, and even measured, by his as- 

 sistant, Olav Hiorter, in March, 1741, Celsius wrote in the summer of the same 

 year to Graham, to invite him to join with him in examining whether certain ex- 

 traordinary perturbations, which the horary march of the declination needle 

 underwent from time to time at Upsala, would also be observable at the same 

 time in London. Simultaneity of disturbance would, he said, afford proof of the 

 cause of perturbation extending over considerable spaces of the Earth's surface, 

 and not being occasioned by accidental local influences. (Celsius, in Svenfka 

 Vetenskaps Academiens Handlingar for 1740, p. 44; Hiorter, in the same work 

 for 1747, p. 27.) When Arago had become aware that the magnetic perturba^ 



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