412 Determination of the Earth's Horizontal Magnetic Force. 



P and Q, though apparently not given explicitly by Lamont, could be 

 easily deduced from his general formulae, see especially his pp. 44 and 

 45. Dr. C. Borgen, in 'Terrestrial Magnetism/ October, 1896, pp. 

 176-190, carries Lamont's formulae somewhat farther. The position 

 taken by the two magnets in the deflection experiment is that termed 

 by Borgen "Erste Lamont'sche Hauptlage." According to Lamont's 

 formulae, the relative importance of Q, and the values of /'//, for which 

 P and Q vanish, must vary according to the law of distribution of free 

 magnetism. At the same time, it would appear that so long as the law 

 of distribution is the same for the two magnets, the conditions for P 

 vanishing is hardly likely to depart much from 



V/l = V2/3. 



The idea adopted in the text of assuming " poles," and proceeding on 

 the hypothesis 



2X/1 = constant, 



is one of which Lamont says on his p. 45, " Dies ist ein sehr niitzlicher 

 Mittel um approximativ den Werth der hoheren Glieder zu bestim- 

 men . . . ." Thus, though not new, as I had supposed, it has the 

 advantage of being recommended by one of the leading pioneers of 

 terrestrial magnetism.] 



§ 47. The exact symmetry in the position of the two magnets which 

 is postulated in obtaining the formula for the deflection is rather to be 

 hoped for than expected. For instance, the magnetic centre of the 

 mirror magnet is hardly likely to lie exactly on the prolongation of the 

 magnetic axis of the collimator magnet, nor is the magnetic axis of the 

 mirror magnet likely to be exactly normal to the mirror. I have, how- 

 ever, no experimental data bearing on either of these points. 



§ 48. The present paper may seem premature, in view of the number 

 of questions which it raises without finally answering. To wait, how- 

 ever, until experiment had given a final answer to all the questions 

 would simply mean shelving the whole subject indefinitely. Many of 

 the questions could be satisfactorily dealt with only by elaborate experi- 

 ments. Such experiments are hardly likely to be carried out at any 

 public institution, within a reasonable time, until the necessity for them 

 has been clearly demonstrated. 



The investigations embodied in this paper have extended over 

 several years, and the results obtained are likely, I think, to be of 

 immediate use. 



The unifilar magnetometer is not the only instrument capable of 

 measuring the horizontal force. Induction coil methods have of late 

 years been introduced and advocated by eminent authorities. The 

 question of the relative merits of magnetometers and induction coils is 

 very likely, I think, to come to the front in a few years, and it is hardly 



