C. GENERAL PHYSICS. 153 



whenever, in the course of his researches, Gaugahi desires to 

 demagnetize a bar of iron, he submits it to the action of a 

 series of alternating currents whose intensity decreases grad- 

 ually and slowly. He supposes that the same method would 

 serve to demagnetize steel. 6 JB, November, 1873, 704. 



SECULAR CHANGES IX THE EARTH'S MAGNETISM. 



Mr. Schott, of the Coast Survey, has made an examination 

 of the secular changes in the magnetic elements, based on all 

 the observations taken at Washington since 1790. He finds 

 that the magnetic declination varies in a periodical manner, 

 such as will cause it to return to its present value in about 

 two hundred and forty years. The dip of the needle is now 

 slowly diminishing, and has continued to do so since 1840, 

 its annual change being very nearly uniform. The total 

 magnetic force is very slowly increasing, although at present 

 it is sensibly nearly stationary; it reached its minimum 

 about twenty-two years ago, and, after having increased un- 

 til the present time, is probably now about to diminish. 

 The hypothesis that the observed secular changes are the ef- 

 fect of thermal changes in the earth's crust, manifesting 

 themselves as a disturbance in the distribution of terrestrial 

 magnetism, seems to the author a plausible one. These 

 thermal changes must be considered to have a slow rate, 

 but operating on a vast scale, explaining the similarity of 

 secular change extending over thousands of miles, and going 

 on, perhaps, for hundreds of years. They appear to be of a 

 mixed, progressive, and periodic character. Thus the influ- 

 ence which produced the increase of the magnetic west dec- 

 lination on our Atlantic coast was first recognized in the 

 Northeast, extending itself in time toward the Southwest. 

 Report of the United States Coast /Survey, 1870, Appendix, 

 No. 14. 



MOLECULAR THEORY OF MAGNETISM. 



Mr. George Gore has communicated to the Royal Society 

 his conclusions in reference to the question as to whether, in 

 the case of two parallel wires conveying electric currents, 

 the attractions and repulsions are between the currents 

 themselves, or the substances conveying them ; and he con- 

 cludes that these forces are not exerted between the currents, 



G2 



