MAGNETIC SURVEY OF GREAT BUITAIN. 185 



seems to be fully accounted for by the imperfection of the 

 instrument used by me in the statical observations, the eifect 

 of the magnetism of the limb (page 106 et seq.) being in this 

 case uncorrected. 



The equations of condition afford the means of deducing 

 the weights of the preceding results, on the supposition that 

 there is no constant error. But as this cannot be supposed, 

 we are left to a certain extent unguided. On the whole, we 

 shall probably be not far from the truth in assigning equal 

 weights to each of the former results, notwithstanding the 

 disparity in the number of stations. The following are the 

 mean values thus deduced : 



L = 1-0263, M = + -000087, N = + -000061. 



Accordingly, the probable value of the intensity at the central 

 station (lat. = 53° 21', long. = 8° C) is 



1 -05^63. 

 And from the mean values of M and N we obtain, for the di- 

 rection of the isodynamic line passing through that station, 



M = - 35° C; 

 and for the rate of increase of the intensity in the direction 

 perpendicular to that line, _ 



r = -000106. 



In order to reduce the intensity results of the present survey 

 to absolute measures, it is only necessary to determine the ab- 

 solute intensity of the magnetic force at some one of the base 

 stations, according to the method of Professor Gauss. This 

 will be done, ere long, in Dublin; and it is therefore important 

 that the ratio of the intensities in Dublin and London (with 

 which latter station all the others are compared) should be ac- 

 curately known. 



For the determination of this ratio we have abundant mate- 

 rials in the present memoir. The ratio of the horizontal inten- 

 sities in Dublin and London, as deduced from the first series, 

 was found to be -9399 ; the result being equivalent to the mean 

 of eleven distinct comparisons. If we combine with this the 

 result obtained by Captain James Ross, namely, -9383, the 

 mean value of the horizontal intensity in Dublin is found to be 



-9398; 

 the horizontal intensity in London being vmity. But the dip in 

 London corresponding to the mean epoch of these observations 

 (the 1st of January, 1837) is 69° 19'-6; and that in Dublin is 

 '71° 1''2 ; wherefore the total intensity in Dublin is 

 ' 1-0201, 



the total intensity in London being vmity. 



Again, we have found that the intensity in Dublin, as de- 

 duced by the statical method from the observations made by 

 Major Sabine and myself, is expressed by the number 1-0197, 



