1373.] On the Magnetic Survey of Belgium in 1871. 165 



being, as they are, the culmination point of the group of Imparidigitata. 

 The reduction of the horse foot, however, is not fully accomplished yet ; 

 to attain this, the styliform metatarsals and metacarpals (the second and 

 fourth) hare to be lost. 



II. "Magnetic Survey of Belgium in 1871. " By the Rev. S. J. 

 Perry. Communicated by Sir Edward Sabine,, K.C.B. ; F.R. S. 

 Received January 24, 1873. 



(Abstract.) 



The magnetic observations which furnished the results contained in 

 this paper were made during the autumn months of 1871. 



The instruments used and the methods adopted were almost identical 

 with those employed in previous magnetic surveys of France. 



The dip was observed by Mr. W. Carlisle, magnetic assistant of Stony- 

 hurst Observatory, and the rest of the observations were taken by the 

 Rev. S. J. Perry. 



This new series of determinations of the terrestrial magnetic elements 

 was rendered the more necessary, as preceding observers had chosen very 

 few stations in Belgium, and as the curvature of the isodynamics and 

 isoclinals in Dr. Lamont's maps of Belgium, Holland, and North-west 

 Germany indicated a very considerable disturbing cause in the first- 

 named country. 



The values obtained in 1871 are a strong confirmation of the suspicions 

 of irregularity to which former observations had given rise ; for although 

 the lines of equal dip, declination, and horizontal force bear a sufficiently 

 close resemblance to those of neighbouring countries, there is evidence of 

 much disturbance ; and when the values of the dip and horizontal force 

 are combined, the isodynamics show clearly that the coal-measures, which 

 stretch completely across the south-east portion of Belgium, exercise a 

 strong disturbing influence. This local magnetism might be incapable of 

 producing more than a decided curvature of the isodynamics of an ex- 

 tended tract of country ; but when all the stations of observation are 

 situated within narrow limits, the perturbation completely masks the 

 normal direction of the lines. 



The following is a complete list of the magnetic elements observed at 

 the different stations, and reduced to the common epoch of January 1, 



1872 :— 



Station. Declination. Dip. Horizontal force. Intensity. 



Aix-la-Chapelle .... 16-464 66-637 4-0064 10-1025 



Alost 17-349 67-210 3-9518 10-2016 



Antwerp 17*489 66-999 3-9296 10-0559 



Arlon 16-398 65-907 4-1175 10-0857 



Bruges 17-938 67-155 3-8950 10-0321 



p2 



