ON THE MAGNETIC SURVEY OF SCOTLAND. 



181 



In determining the total force by this method, a constant is made use of, 

 which is best found at the base station by comparison with a unifilar. A 

 priori, there is no reason for supposing that this constant will change, so 

 that it only requires to be determined once for all ; yet, in the instrument 

 used in the Scotch survey, there is reason for supposing that a change in the 

 value of the constant took place between the first year and the second. 



This will be seen by the following Table, which exhibits the values of the 

 total magnetic force given by the circle at Kew at different periods, the same 

 constant being used throughout : — 



Table XII. 



From this Table it will be seen, that while the instrument with the face of 

 the deflected needle to the east gave on the 6th of August, 1857, a reading as 

 high as 103 10, on the 15th of October of the same year this had fallen as 

 low as 10-252, from which it gradually rose again, until in February, 1859, 

 it had attained the same value as at first. 



The constant seems to have been chosen to make the first two observa- 

 tions of August 6 and 7, 1857, give a value for the total force = 10-310, hut 

 from a table already given, its most probable value about this period was found 

 to be 10299, or nearly 10-300. Let us, therefore, deduct 001 from the values 

 of total force obtained by this method during the first year's survey, for in 

 these the same constant and arrangement of instrument were employed, which 

 gave the value 10 - 31 at Kew. We thus obtain the following table of com- 

 parison between the results obtained during the first year by the two me- 

 thods : — 



