PEEFACE TO PAKT II. 



I PROPOSE to give in this Preface a short account of Professor John 

 Couch Adams's Theory of Terrestrial Magnetism, and of his determination 

 of the Gaussian magnetic constants. This work was first taken in hand 

 by him just fifty years ago, not long after the discovery of the planet 

 Neptune. I find from his papers that the earliest work which he did on 

 this subject was begun in the year 1849, and that he was led to it by the 

 study of the translation of Gauss's Memoir on the Theory of Terrestrial 

 Magnetism given in Taylor's Scientific Memoirs which was published in 

 1841. Gauss himself says in that memoir that he was stimulated to under- 

 take the work on the publication of Sabine's map of the total intensity 

 in the seventh Report of the British Association (i.e. in 1837), but that 

 the data were very scanty for the accurate determination of the magnetic 

 constants. For their accurate determination data should be supplied from 

 accurate observations of magnetic declination, horizontal intensity, and dip, 

 taken at stations uniformly distributed, as in a network, over the surface 

 of the Earth. 



Not only fifty years ago, when Gauss wrote, but even to the present 

 day, the progress made in the theory of terrestrial magnetism has suffered 

 from the lack of data derived from observations, because even now there 

 are very few magnetic Observatories in existence, and those few are for 

 the most part grouped very close together, leaving other parts of the Earth, 

 and especially the southern hemisphere, almost entirely wanting in the facts 

 of observation without which all theories can be but visionary. 



In his calculations on the magnetic potential of the Earth and on the 

 theoretical expression of the magnetic components X, Y and Z, to the 

 north, to the west, and vertically downwards respectively, Gauss expressed 



A. n. b 



