1883.] On Irregularities in Magnetic Inclination. 



9 



the area of attractive matter in Mull is near the upper part of Loch 

 Tuaclh or Loch Na Keal ; that is to say, at about the spot indicated 

 by Dr. Balfour Stewart as its probable place. This spot is certainly 

 very close to, and may indeed on a more complete examination turn 

 out to be identical with the locality where Profesor Judd places the 

 great volcanic vent in the island. 



The observations, moreover, show that great as may be the 

 absolute mass of the magnetic matter in Mull, its influence extends to 

 no very great distance from the island. The observations in Loch 

 Aline to the north-east, and in Loch Scridain to the south-west, 

 prove that the masses of igneous rocks, as these are constituted in 

 Mull, may be approached to within a very few miles without 

 appreciable effect on, the needle. The determination of the normal 

 direction of the isoclinal line in the neighbourhood of Mull is not 

 likely therefore to be materially affected by this area of attractive 

 matter. It is almost certain for instance, that the area was without 

 sensible influence on the observations at Shuna, Carron Point, Loch 

 Corrie, and Loch Ailort. At Loch Drumbuy the influence of the 

 Morvern basalt is appreciable, whilst at Fishnish the effect of the 

 horizontal component of the attractive force is very marked. 

 Although this place is only, about five miles in about the normal 

 direction of the isoclinal from Kinloch Aline, there is a difference of 

 42' in the dip, an amount of course far in excess of any possible error 

 of observation. 



Our observations lastly afford some indication of the probable 

 amount of the secular change in the inclination in these parts. So 

 far back as 1836*5, Sir. Edward Sabine found the dip at Artornish 

 to be 72° 43' ; this place is close to Kinloch Aline where we this year 

 found the dip 71° 9' ; this is equal to an average yearly decrease of 2'. 

 At our station in Shuna* we observed the dip 70° 54 /- 5 ; the inter- 

 polated value for 1858 as obtained from the chart accompanying 

 Professor Stewart's report is 71° 45' ; this too is exactly equal to a 

 yearly decrease of 2'. It is worthy of note that this value is identical 

 with that adopted by Sir Edward Sabine in the reduction of all the 

 Scotch observations made prior to 1858 to the mean epoch of 1842'5 

 ("Contributions to Terrestrial Magnetism," No. XII, "Phil. Trans.," 

 1870, p. 265). 



Our observations are, of course, too few in number, and are distri- 

 buted over too restricted an area to allow us to draw any definite 

 conclusions as to the direction of the isoclinals at the present time, 

 but it is almost certain that this direction will now be found to be 

 very different from that observed by Mr. Welsh in 1858. It is to be 

 hoped that before very long another magnetic survey of the whole of 

 the British Isles will be set on foot. So far as Scotland is concerned 

 the present time would appear to be very opportune for a new survey. 



