Observations on the Direction and Intensity of the Terrestrial 

 Magnetic Force in Scotland. By Major Edward Sabine, 

 B.A., F.R.S., &;c. 



[With a Plate.] 



J- HE observations, which form the subject of the following pa- 

 per, were made dm-ing a leave of absence with which I was in- 

 dulged from military duty in Ireland, in the summer of 1836. 



I was indebted for the opportunity of extending them to several 

 stations in the Western Islands, to the interest felt in the subject, 

 and to the private friendship of James Smith, Esq., of Jordan- 

 hill, President of the Andersonian Institution at Glasgow, who 

 devoted a cruise of his yacht to the purpose. 



I. Observations of Dip. 



The needle employed in making the observations of dip is the 

 one characterized as S (2) in the account of the Magnetical Ob- 

 servations in Ireland [Fifth Report of the British Association, 

 pp. 141 — 149). It was made for me by Robinson, of Devon- 

 shire-street, London, to be used on Professor Lloyd's principle 

 of determining both dip and intensity by the same needle. Its 

 length is 11| inches. The only peculiarity in the apparatus in 

 which it is used is, that^the agate planes of support are rendered 

 horizontal by a detached circular brass plate, groimd for the pur- 

 pose, and carrying a small spirit level. The plate is placed on 

 the planes themselves, and the foot-screws are adjusted until the 

 level remains stationary whilst the plate is turned in every di- 

 rection ; and it is then removed. There are also levels attached 

 permanently to the circle, which serve to detect any change which 

 might subsequently take place in the horizontality of the planes 

 during the course of the observation. The mode of observation 

 of the dip is the same as usual. The number of readings em- 

 ployed in each result is noted in the tables, the reading of each 

 pole being considered as a distinct reading. The needle was re- 

 moved from its supports by Ys, and lowered afresh between each 

 pair of readings ; and the face of the circle and of the needle were 

 changed round frequently in the course of each determination. 



In a needle on Professor Lloyd's principle, the poles are not 

 changed in each observation of the dip ; consequently a correc- 

 tion is required to be applied to all the results obtained with it 



VOL. V. — 1836. H 



