ON THE MAGNETIC SURVEY OF SCOTLAND. 167 



On some Results of the Mugnetic Survey of Scotland in the years 

 1857 <tnd 1858, undertaken, at the request of the British Asso- 

 ciation, by the late John Welsh, Esq., F.R.S. By Balfour 

 Stewart, A.M. 



The much-lamented death of Mr. Welsh, who laboured in science so well 

 and so earnestly, and the last work of whose life was the completion of the 

 observations of the Magnetic Survey of Scotland, has imposed upon the author 

 the less arduous task of reducing those observations. 



It is now somewhat more than twenty years since the first Magnetic 

 Survey of the British Islands was made, the results of which are recorded by 

 General Sabine in the Report of the British Association for 1838. 



The General Committee at the Meeting held at Cheltenham in 1856, 

 feeling that the time had arrived when another survey of these islands would 

 be desirable, requested General Sabine, Prof. Phillips, Sir James C. Ross, 

 Mr. Robert W. Fox, and Rev. Dr. Lloyd, to undertake its repetition. It 

 was ultimately resolved that Mr. Welsh should proceed to Scotland, and 

 the Admiralty kindly granted £200 in aid of his expenses. 



During the summer and autumn of 1857 Mr. Welsh performed the first 

 instalment of his task, confining himself to stations in the interior of Scot- 

 land and on the east coast. In the same season of 1858 he completed the 

 survey, by undertaking the west coast, the Hebrides, and the Orkney and 

 Shetland Isles. This involved much personal fatigue and a great number 

 of observations, all of which were executed with the utmost possible accuracy 

 and scientific attention to details. 



More was clone for Scotland in this survey than in that of twenty years 

 ago. In the interval between the two surveys, improvements had been 

 made in the dip-circle and in the apparatus for measuring the total mag- 

 netic force. These improvements were of course adopted in the instru- 

 ments employed in the late survey ; and, furthermore, observations of decli- 

 nation were made, — a thing which had not been previously attempted. The 

 survey thus divides itself into three parts : the first comprising the Observa- 

 tions of Dip ; the second those of Total Force ; and the third those of Decli- 

 nation, which will be discussed in order. 



Division I. — Dip. 



The dip-circle (No. 23) was made by Barrow. Two needles were em- 

 ployed, each 3£ inches long. The axle of the needle rests on two agate 

 planes, and its position is concentric with, but behind (as regards the observer) 

 the vertical divided circle on which the inclination is read. A moveable 

 arm, concentric with this circle, has two microscopes attached to it, the di- 

 stance between them being 3^ inches, so that either extremity of the needle 

 may be brought into the centre of the field of the corresponding microscope. 

 The extremities of this moveable arm form verniers which bear upon the 

 vertical circle, and by means of which the position of the needle may be 

 very accurately determined. In November 1857, the following observations 

 were made with this instrument in different azimuths : — 



