162 



N 



where near the west coast of England the mean annual secular change 

 in the twenty years has been greater, and near the east coast less than 

 its mean value at Kew ; showing that the general direction of the 

 isoclinal lines more nearly approaches a parallelism to the equator now 

 than it did twenty years ago. The ascertainment of the exact value 

 of the secular change at a particular locality by a well-conducted 

 system of periodical observations is the duty of a magnetic observa- 

 tory ; the direction of the magnetic lines passing across a country 

 is supplied by magnetic surveys ; which, for that purpose, ought to 

 be repeated from time to time, as they have now been in this country, 

 at intervals of perhaps twenty or twenty-five years. 



It has been imagined that the secular changes of the magnetic ele- 

 ments may be due to some alteration taking place either in the dis- 

 tribution or in the condition of the materials in the interior of the 

 globe. But the regularity and uniformity with which the secular 

 magnetic changes continue through long intervals of time, together 

 with their sudden periodic reversals, and their corresponding fea- 

 tures in the northern and southern hemispheres, which add greatly 

 to the apparent consistency and systematic character of the whole as 

 parts of a uniform general system, wear more the aspect of effects 

 of some yet unascertained cosmical cause. One of the British Colo- 

 nial Observatories, St. Helena, having the advantage of both a large 

 secular change and a small amount of magnetic disturbance, has 

 afforded a very striking example of the great regularity with which 

 the secular change takes place, maintaining a steady uniformity, 

 traceable not only from year to year, but from month to month, and 

 even from week to week ; so that it is not too much to say that, 

 from observations made during a single fortnight, an annual secular 

 change which has existed almost without variation for more than a 

 century, may be ascertained and measured with very considerable 

 precision. (Magnetic Observations at St. Helena, vol. ii. p. ix.) 



March 21, 1861. 



Major-General SABINE, R.A., Treasurer and Vice-President, 

 in the Chair. 



Professor Herman Helmholtz, elected a Foreign Member, was 

 admitted into the Society. 



