the Rock on the Summit of Arthur s Seat. 287 



to the top of Arthur's Seat, and, in order to look out for the 

 summits of some of the mountains in the Highlands, I deter- 

 mined their bearing previously, so that, by means of Schmalcal- 

 der's surveying compass, I could find their directions, referred to 

 some near fixed object, and would be enabled to examine the 

 horizon in the proper line of bearing, whenever the weather was 

 clear and highly favourable for distant views, required in such 

 cases. On placing the compass, however, on what was thought 

 a convenient place of the rock, I was struck with the remark- 

 able deviation of the sight vane of the compass from the direc- 

 tion in which I knew Benlomond should be found. On re- 

 moving the compass to a different position, the card was com- 

 pletely reversed, the north pointing to the south, and the south 

 to the north. The compass was then carried to different points 

 of the rock, where it still showed remarkable anomalies, the 

 north point of the card deviating sometimes to the west, some- 

 times to the east, and at other times it stood nearly in the me- 

 ridian. It was then resolved to make a more complete set of 

 observations on some future day. 



On the 12th, the following table, derived from a mean of 

 two sets of observations by different compasses and different 

 observers, was completed. The one with Schmalcalder's com- 

 pass, employed by myself, the other with a new surveying 

 compass by Mr Adie, and used by my friend Mr James Trotter, 

 who assisted me in making the necessary observations. The 

 angles are the bearings of the dome of the New Observatory, 

 and are marked to correspond with the same letters in the ac- 

 companying plan of the small rock on the summit of the hill, 

 on which the observations were made. 



To obtain the true direction of the meridian, Mr John Adie 

 found, from observations with the new astronomical circle, on a 

 stone pillar terminating in the dome of the Observatory, that 

 the highest point of the rock bore S. 48° 11' E. 



Also the line A Q, 31 feet in length, from which ordinates 

 were drawn to the different points where the observations were 

 made, formed an angle of 98°, with the magnetic meridian at 

 the point Q, that is, the point A bore N. 98° E. from Q. 



Hence the position of the north point of the compass at each 

 point of observation may be found, and a few of the more pro- 



