ON COMPARING AND REDUCING MAGNETIC OBSERVATIONS. 67 



APPENDIX. 



I. Letter from Captain Creak to Professor Stewart. 



Eichmoncl Lodge, Kidbrooke Park Road, 

 Blackheath : Ajjril 26, 1886. 



Dear Professor Stewart, — In tlie appendix accompanying tbe last 

 Report of the Committee on Reducing and Comparing Magnetic Obser- 

 vations, so many valuable suggestions are made by various well-known 

 magneticians that I feel there is little left for me to add. 



I have long noticed the difficulties attending Sabine's method of 

 separating the disturbances from the normal values of the solar diurnal 

 variation of the declination. It has done good work in the past, but 

 now the question has arisen, Has a better been proposed ? I think'that 

 adopted at the Greenwich Royal Observatoiy is better, and that the 

 whole of the Greenwich methods of reduction, as set forth in the pub- 

 lished volume of ' Magnetic Reductions ' of 1883, invite the attentive 

 consideration of the Committee, with a view to their adoption as a whole 

 or in part. 



_ I am, however, disposed to think that the method proposed by M. H. 

 Wild, of ascertaining ' the noi-mal daily path of the magnetic elements.' 

 has niuch to commend it, and is rather less open to the possibility of 

 individual bias than that of Greenwich. 



In recalling to the notice of the Committee Gauss's valuable memoir 

 ' On the General Theory of Magnetism,' I consider Dr. Schuster has done 

 excellent service. Possibly the prospect of formidable computations has 

 prevented Gauss's treatment of magnetic observations from beino- hitherto 

 adopted, but if, as Dr. Schuster proposes, by selecting stations °the com- 

 putations may be reduced within comparatively easy limits, I would 

 suggest some such course as follows : — 



(1) That the selected stations be fixed observatories provided with 

 the usual magnetogi'aphs of like pattern. 



(2) That the several Superintendents be invited to make a series of 

 observations for a year or more of the solar diurnal variation of the three 

 magnetic elements, according to a method to be decided by the Com- 

 mittee, with a view to their being treated after the method of Gauss. 



(3) That Earth currents be made, as far as possible, a subject of 

 observation at each observatory. 



In thus advocating the application of Gauss's method of calculation to 

 the variations of the magnetic elements, I apprehend that the most im- 

 portant immediate result will be the settlement of the question whether 

 the causes are situated above or below the surface of the Earth, and 

 consequently we shall thereafter be better instructed as to the path we 

 should follow in future observations. 



Although in these suggestions only future observations have been 

 considered, it is not in forgetfulness of the large and valuable series 

 already obtained, which all must wish to see made generally available by 

 being rendered in a common form. 



I remain, yours truly, 



Ettrick W. Creak. 



II. Letter from Senhor Capello, Lislon, to Professor Stewart. 

 Lately in studying the diurnal variation of the tension of vapour at 

 Lisbon, I have been struck with the great similarity of the course of this 



