78 Mr William Swan on a 



On a Simple Variation Compass. By William Swan, 

 F.R.S.E., F.R.S.S.A.* 



About two years ago, my friend Mr John Adie communi- 

 cated to the Royal Scottish Society of Arts the description of 

 a new variation compass. His instrument, which is intended 

 to be used along with an ordinary theodolite, was devised for 

 the purpose of ascertaining the magnetic meridian with greater 

 accuracy than is attainable, either by the use of the compass 

 usually attached to theodolites, or by employing the more ordi- 

 nary forms of the azimuth compass. 



Mr Adie's very elegant invention is described in the Trans- 

 actions of the Royal Scottish Society of Arts, vol. iv., p. 138. 

 It consists of a delicately-suspended compass-needle, inclosed 

 in a tube furnished with collars, which are placed in the Ys 

 of the theodolite, the telescope having been previously re- 

 moved. The ends of the needle, which are brought to fine 

 points, are nearly in contact with finely divided glass dia- 

 phragms ; and the needle being viewed through the dia- 

 phragms by powerful eye-pieces, has its ends accurately re- 

 ferred to those divisions. It is easy to see how, in this man- 

 ner, the axis of the tube with its collars, — which, when placed 

 in the Ys, is coincident with the axis of the theodolite tele- 

 scope occupying that situation, — can be placed parallel to the 

 axis of the needle ; and the reading on the horizontal limb of 

 the theodolite corresponding to magnetic north may be ob- 

 tained. 



From actual trial, I was so much satisfied of the excellence 

 and utility of Mr Adie's instrument, that I felt desirous of 

 having something of the same kind applied to a Kater's alti- 

 tude and azimuth circle in my possession ; but as the telescope 

 of that instrument, unlike that of the ordinary theodolite, 

 does not admit of being removed, I was obliged to adopt an 

 arrangement totally different from Mr Adie's. 



The instrument I devised was constructed for me by Mr 

 Adie in the autumn of 1852 ; and I now describe it, in the 

 hope that it may be useful to persons who, possessing instru- 



* Read before the Royal Scottish Society of Arts. 



