1868.] 377 [Chase. 



with such modifications as seem to be warranted by the fore- 

 going considerations. Ahhoiigli I can hardly imagine the pos- 

 sibility of essential change in any particnlar, 1 submit it as a 

 merely provisional statement, subject to such future amend- 

 ments as may be required by the progress of discovery, 



1. The earth is an electro-magnet, magnetized by currents 

 which are excited by the sun and by the earth's rotation. 



2. Terrestrial magnetism is subject to a variety of disturb- 

 ances, some of which are periodical, others irregidar and oc- 

 casional. 



3. The principal periodical disturbances vary: 1st, with the 

 solar hour ; 2d, with the season of the year ; 3d, Avith the rela- 

 tive distance of the earth from the sun ; 4th, with the rapidity 

 of the earth's orbital motion ; 5th, with the rapidity of change 

 in solar declination ; Gth, with the absolute hour, in reference 

 to magnietic meridians in the Pacific and Atlantic oceans ; tth, 

 with the lunar hour ; 8th, with the lunar declination ; 0th, with 

 the position of the principal planets (sun-spot period); 10th, 

 with change of climate (secular variation). All these disturb- 

 ances (the last, perhaps, excepted), appear to be transmitted 

 with the speed of gravity, and they may all be grouped under 

 two heads: 1st, change of relative position; 2d, change of 

 relative velocit}'', between the disturbing and the disturbed 

 bod}^ 



4. The principal occasional, or irregular disturbances, seem 

 to be dependent upon, 1st, irregular variations in the light or 

 heat transmitted from the sun ; 2d, similar variations in the 

 diathermancy or transparency of the earth's atmosphere ; 3d, 

 local accumulations of heat or cold ; 4th, electric and other lo- 

 cal meteoric changes ; 5th, atmospheric or terrestrial agitation 

 (c3'clones, earthquakes, volcanic eruptions, &c.). These dis- 

 turbances are, of course, transmitted with velocities varying 

 from that of light or of electricity to that of sound-waves, or 

 even to that of still more sluggish vibrations. 



5. The periodical and the occasional disturbances are all 

 modified by mutual interaction. The phenomena are thus com- 

 plicated, and the difficulty of satisfactory investigation is largely 

 increased. 



6. The sources of disturbance are, therefore, multiform ; 

 some are celestial, and others terrene ; some cosmical, and 



VOL. X. — 3a 



