MAGNETIC SURVEY OF GREAT BRITAIN. 187 



results. But the values of r, or the rate of increase of the in- 

 tensity corresponding to equal geographical spaces, differ con- 

 siderably, and give a decided indication that the spaces between 

 the isodynamic lines are less in Scotland than in England. If 

 we examine the partial results obtained in the two countries by 

 the different observers, and by the different methods of obser- 

 vation, we perceive that all the series are consistent in this in- 

 dication. The lines which are selected for representation in the 

 map are those of unity (passing through London), of I'Ol, 1"02, 

 and 1-03 : the mean distance between the lines, which thus differ 

 •01 \i\ the values of the intensity they represent, is in England 

 116, and in Scotland 75 geographical miles ; the partial results 

 vary in England from 106 to 1^6 miles, and in Scotland from 

 69 to 94 miles. 



Whatever may be the cause of this difference in the value of 

 r in the northern and southern portions of the island, it is obvi- 

 ously much too great to be taken as a regular part of a general 

 progression; as in its extension towards the N.W. and S.E., the 

 separation between the lines would in the one case be soon ren- 

 dered extravagantly small, and in the other extravagantly great. 



In order to deduce the position of the several isodynamic 

 lines in best conformity with the observations, it is particularly 

 necessary, under such circumstances, to derive each line from 

 those observations only which are in its immediate vicinity ; 

 and thus to reduce within very small limits the effect on each 

 of the rapidly-changing and somewhat uncertain values of r. 

 We require, for this purpose, only its approximate values 

 in the vicinities of the respective lines ; and without entering 

 into nice calculations where we have not a sufficiently satisfac- 

 tory basis, we may provisionally assume these values as follows ; 

 always remembering, that any inaccuracy in the assumption will 

 produce an opposite effect on the deductions from the observa- 

 tions which are on either side of each isodynamic line, and that 

 such opposite effects will counterbalance each other in the mean 

 position assigned to the line. 



Approximate values of r in England and Scotland, in the vici- 

 nity of the several isodynamic lines : 



Line of 1-0 , r= -00008 

 .... 1-01 ; r = -00009 

 .... 1-02; r = -00011 

 .... 1-03; ?• = -000135 



The mean value of r in Ireland, derived from the several 

 series in that country, is -000104 or -000106, (page 185,) which 

 corresponds so nearly with the value which might be interpo- 

 lated from the results in England and Scotland for the latitude of 

 the central geographical position in Ireland, that we may safely 

 take "00010 as a general value for the Irish deductions. 



