ON THE MAGNETIC SURVEY OF SCOTLAND. 



185 



representing approximately the declination at the epoch of January 1, 1858. 

 It will not do so exactly, because it will correspond to the middle and not to 

 the beginning of January ; but the difference will be so trifling, that we may 

 suppose the correspondence to be exact without further refinement. 



Now, by means of the traces of the magnetograph we can find the difference 

 between the declination at Kew at any moment of 1858 and that corre- 

 sponding to the epoch of January 1, 1858, as above defined. And this we 

 can do even if a considerable magnetic disturbance be going on at the mo- 

 ment we fix upon for comparison with the epoch, because this disturbance 

 is registered by the magnetograph, and it may therefore be measured and 

 allowed for. 



Now, the moment at which the needle was observed at any station in Scot- 

 land in the year 1858 has been recorded ; if, therefore, we suppose the same 

 magnetic changes to take place simultaneously in Scotland and at Kew, the 

 indications of the Kew magnetograph will afford us the means of reducing 

 accurately the observations of declination taken in Scotland in 1858 to our 

 epoch January 1, 1858. This method has been pursued with these obser- 

 vations. 



The following Table exhibits the most probable value of the absolute 

 declination at Kew corresponding to 1st January, 1858, the method of reduc- 

 tion to epoch being that now mentioned : — 



Table XV. 



