Chase.] 154 [October. 



17. Inclining the disc towards N. W. and S. E., the needle nioves 

 towards the east, whether the opening of the disc is towards the 

 noi'th or towards the south. 



18. If the inclination is towards N. E. and S. W., the motion of 

 the needle is westerly. 



Experiments 14 to 18 are perhaps the most interesting and im- 

 portant of the entire series. Exp. 14 represents the direction of the 

 gravitation-currents that tend to restore the equilibrium which is con- 

 tinually disturbed by the thermal radiation of the northern hemi- 

 sphere; while Exp. 15 represents the like direction in the southern 

 hemisphere, and all the ex.periments demonstrate that in the. daily 

 ina(jnetlc fluctuations, the, motion of that end of the needle which is 

 nearest to the equator should follow, •precisely as observation shows 

 that it does follow, the direction of the sun's meridional influence. 



b. Axis in magnetic equator. 



19. When the disc is in the meridian, the current-polarity and the 

 magnetic polarity coincide. 



20. If the disc is in the equator, with the opening to the east, 

 when the top of the ring moves towards the north the needle declines 

 to the east. 



21. But when the motion of the ring is towards the south, the 

 current-polarity is reversed, and the needle declines to the west. 



22. The reversal of the disc, placing the opening to the west, like- 

 wise reverses the polarity, the declination being east when the ring 

 moves south, and west when the ring moves north. 



23. The disc being inclined to N. W. and S. E., the needle de- 

 clines towards the west (the action in Exp. 17 being reversed). 



24. Changing the inclination of the disc to N. E. and S. W., the 

 declination changes to east (reverse of Exp. 18). 



From Exp. 19 to 24 we may infer that, in consequence of the 

 action of the trade-winds, combined with the greater radiation of the 

 northern hemisphere, there should be a constant eastivard tendency of 

 the magnetic declination, such as is indicated by the secular variation 

 of the needle. 



c. Axis ^h° from, magnetic meridian. 



25. When the disc is in or near the meridian, there is a slight 

 tendency, in the extremity of the needle which is under the opening 

 of the disc, to follow the direction of the top of the ring. In all 

 other positions, the needle declines from the axis. 



