MAGNETIC SURVEY OF GREAT BRITAIN. 



157 



In the discussion of these observations in the 6th Report of 

 the British Association, I have adverted to the frequent in- 

 fluence of the igneous rocks in Scotland in producing vi^hat 

 may be termed statioti error. In the table in page 20 of that 

 Report, the intensities observed at Tobermorie, both by the 

 statical and horizontal methods, are shown to have been af- 

 fected, apparently by an error of this nature, to a degree much 

 exceeding that of the results at any other station. In com- 

 bining the results of both methods, therefore, for the values 

 of X, y, &c., I have thought it right to omit altogether the inten- 

 sities at Tobermorie. We have, therefore, the statical results 

 at nineteen stations to combine by the method of least squares, 

 vi'hence we obtain the following values : a; = + •000083 ; 

 y = - -000107 ; e^ = - 52° 15'; r^= '000136. The mean geo- 

 graphical position is in latitude 56° 22' N. and longitude 

 4° 01' W. 



Captain Ross's Observations. — These were made "with two 

 needles, RL (3) and RL (4), on Professor Lloyd's principle, used 

 in Captain Ross's six-inch circle. One of these needles (R L 3) 

 appears to possess the peculiar property of preserving its mag- 

 netism unchanged in different temperatures, requiring no re- 

 duction to a mean temperature. Table LVII. contains a series 

 of experiments with it, made by Captain Ross, by which it will 

 be seen that in differences of temperature, including the whole 

 range of natural temperatures to which it is likely to be exposed, 

 the time of vibration of the needle remained unaltered. 



Table LVII. 



Observations to investigate the influence of differences of tem- 

 perature on the time of vibration of Captain Ross's statical 

 needle R L (3). 



The following table. No. LVIII. contains two series of ex- 

 periments of a similar nature, with R L (4), one made by Major 



