1880.] On the Magnetic Inclination in the Azores, 



237 



Scale reading. Wave-length. 



84 459 



85 457 



86 455 



87 453 



88 . 451 



89 450 



90 448 



91 447 



92 . . 445 



93 443 



Scale reading. 



Wave -length. 



94 



442 



95 



440 



96 



. , . 438 



97 



437 



98 



435 



99 



434 



100 



432 



101 . 



430 



102 



429 



103 ... 



, . . . 427-5 



IV. ''Note on the Determination of Magnetic Inclination in the 

 Azores." By T. E. Thorpe, Ph.D., F.R.S. Received No- 

 vember 13, 1880. 



With the exception of a series of determinations made by the 

 officers of the " Challenger " at Ponta Delgada, St. Michael, in 1873, 

 no magnetic observations have, so far as I can learn, been made in the 

 Azores since the time of Captain Vidal's hydrographic survey in 

 1843-4. A visit to these islands during the past summer has enabled 

 me to offer the small contribution to their magnetic history which 

 forms the subject of the present communication. 



Magnetic observations are made with some difficulty in the Azores, 

 on account of the intensely volcanic character of the islands. Con- 

 siderable care was however taken in selecting the stations, and there 

 is no reason to suppose that the observations are affected to any great 

 ■extent by the nature of the soil or rock immediately beneath the 

 instrument or in proximity to it. The places chosen were such as 

 will enable subsequent observers to repeat the determinations on the 

 .same spots. 



The dip- circle employed was Dover 3, belonging to the Owens 

 College, Manchester : I had previously used this instrument in the 

 .course of my magnetic observations along the fortieth parallel in 

 North America (" Proc. Roy. Soc," vol. 30, p. 132), and am again 

 indebted to Professor Balfour Stewart for the loan of it. It was pro- 

 vided with two needles, each 3J- inches long and 0"27inch in maximum 

 breadth. The same precautions were taken to preserve the needles 

 from rust as are described in the communication above referred to, 

 and the method of observation was identical with that previously 

 adopted. In all cases duplicate and independent observations were 

 made with the two needles. 



VOL. XXXI. S 



