Mr Harker, Magnetic Disturbances in the Isle of Skye. 271 



about 10 feet S.W. of the edge of the cliff, is also highly 

 magnetised. Observations of declination were taken at 46 points 

 within a radius of 15 yards of the summit-cairn, and the results 

 are shown on the accompanying ground-plan (PL XL). The de- 

 clination proper to the time and place is about N. 21° W., and 

 the edge of the cliff, which is practically the crest-line of the hill, 

 is thus roughly coincident with the normal magnetic meridian. 

 In order to note the effect of this crest-line we will consider the 

 deflections of the needle from the normal magnetic meridian as 

 observed at points on opposite sides of the line. For brevity we 

 may call deflections to west and east of north positive and 

 negative deflections respectively. Of 28 observations at points 

 to the (magnetic) west of the line, 2 gave the normal declination ; 

 4 gave positive deflections, but never exceeding 2^° in amount ; 

 while 22 gave negative deflections varying up to 13°, with a mean 

 of about 6^-°. The mean for the 28 was a negative deflection of 

 nearly 5°. Of 18 observations at points to the (magnetic) east 

 of the line, 7 were in the neighbourhood of highly magnetic 

 crags, and so gave large and irregular deviations : these, for our 

 present purpose, may be left out of account. Of the remaining 

 11, 3 gave negative deflections, but only of 2°, 5°, and 7°, re- 

 spectively; while the other 8 gave positive deflections varying 

 from 6|° to 30|°, with a mean of 17°. The mean for the 11 was a 

 positive deflection of 11°. It will be noticed that the deflections 

 are higher and also more irregular on the steep than on the gentle 

 slope. The irregularities, however, are not so great as to hide 

 what appears to be the general rule governing the disturbance ; 

 viz. that the north pole of the needle is deflected eastward on 

 the west side of the crest-line and westward on the east side. 



A similar diagram of the behaviour of the compass-needle on 

 the summit of Glamaig was given by Macculloch in a paper 

 published in 1816 1 , and one object of my own observations was to 

 ascertain what changes, if any, had taken place in the interval of 

 more than eighty years. The comparison is not very easily made, 

 for the reason that Macculloch had no topographical map to guide 

 him : he does not tell us how he fixed the points of the compass, 

 and there would also be some small latitude possible in fixing the 

 actual summit-point on the round top of the hill. The only 

 differences that can be certainly verified between the two diagrams 

 relate to places of violent local disturbance. One such is in- 

 dicated on Macculloch's diagram which certainly did not exist at 

 the time of my observations (September, 1898). It is on the 

 smooth side of the hill, about 36 yards from the summit, in a 

 direction a little west of N.W. Here Macculloch marks an arrow 



1 Trans. Geol. Soc, vol. in., plate 3, fig. 1, 



