212 



were first noticed by Mr. Fox Talbot, and were ascribed by Sir Da- 

 vid Brewster to a new property of light, consisting of a peculiar kind 

 of polarity. 



The author shows, that on the undulatory theory, in all cases, a 

 difference of retardation between the two halves of each primary 

 pencil throughout the spectrum may give bands within certain 

 limits ; and that it affords a complete explanation of the phenomena 

 in question. 



March 19, 1840. 



The MARQUIS of NORTHAMPTON, President, in the Chair. 



A paper was read, entitled " Contributions to Terrestrial Mag- 

 netism." By Major Edward Sabine, R.A., V.P.R.S. 



An increased activity has recently been given to researches in 

 terrestrial magnetism, with the definite object of obtaining correct 

 maps of the magnetic phenomena, corresponding to the present 

 epoch, over the whole surface of the globe. To aid these researches, 

 and to facilitate the comparison of the general theory of M. Gauss 

 with the facts of observation, maps have been constructed of the 

 magnetical lines, both as computed by the theory, and as derived 

 from observations already obtained. The theoretical and actual lines 

 of the declination and intensity have thus been represented in maps 

 recently published in Germany and England, as have also the lines 

 of the inclination computed by theory ; but the corresponding map 

 or the latter element derived from observations is yet wanting. The 

 object of the present communication is to supply this desideratum, 

 as far as regards the portion of the globe contained between the 

 parallels of 55° N. and 55° S., and the meridians of 20° E. and 80° 

 W. ; comprising the Atlantic ocean, and the adjacent coasts of the 

 continents on either side. 



The observations chiefly employed for this purpose are two series 

 made at sea ; one by Mr. Dunlop of the Paramatta observatory, in a 

 voyage from England to New South Wales, in 1831 ; the other by 

 Lieut. Sulivan of the Royal Navy, in a voyage from England to the 

 Falkland Islands, and back, in 1838 and 1839. The observation of 

 the magnetic dip at sea, which was commonly practised by the 

 distinguished navigators of the last century, was unfortunately not 

 resumed when the interest in such researches was revived on the re- 

 storation of peace : but it is by such observations only that the lines 

 of inclination can be independently traced over those large portions 

 of the globe which are covered by the ocean. The difficulties which 

 attend the observation, occasioned by the motion and the iron of a 

 ship, require the adoption of several precautions, which it is particu- 

 larly desirable at this time to make generally known. The series of 

 Messrs. Dunlop and Sulivan are discussed in this view ; and the 



