PHILOSOPHICAL SOCIETY OF WASHINGTON. 4t 



the spotted surface of the sun the greater the daily range of the 

 motion of the needle and the less the activity of the sun in pro- 

 ducing sun spots the less this daily magnetic motion. The angu- 

 lar range between the eastern (morning) and western (afternoon) 

 elongations is, for instance, at Philadelphia about 8' on the ave- 

 rage of the year, at Key West, Florida, it is about 5^', during 

 August it is 10|' at Philadelphia, and during November but 6' 

 at the same place, and is nearly double in amount during the 

 maximum of sun spots as compared with the amount during the 

 minimum period. 



The annual variation is a small periodic change in the declina- 

 tion of at most 1^' of arc. 



The lunar inequalities are still smaller in extent, twice each 

 lunar day or during 25 solar hours the magnetic needle is found 

 subject to two oscillations, that is, there are two maxima and 

 two minima with a range between them of about 2t" at Phila- 

 delphia and 38" at Toronto, Can. These may be compared with 

 the moon's tidal action producing two high and two low waters 

 each lunar day, and the magnetic effect may possibly be due to 

 change in the lunar gravitation which brings the terrestrial 

 spheroid twice each day into a state of constraint and release 

 alternately. Possibly this curious effect as well as the solar ine- 

 quality may ultimately depend on changes of heat, which is known 

 to affect the intensity of magnetism. 



Magnetic disturbances or storms may occur at any time, though 

 they cannot be predicted, yet when treated by the established 

 method they are found subject to various laws. They consist of 

 sudden and sometimes of great deflections or of irregular wavy 

 motion and may continue for a day or even for several days; they 

 are frequently accompanied by auroral lights and by strong elec- 

 tric earth currents. They likewise depend on the condition of 

 the sun with respect to spots. 



The secuZar c/iangre of the declination is supposed to be of pe- 

 riodic character, requiring centuries for its full development ; the 

 motion, may be compared with that of an oscillating pendulum 

 which comes to rest momentarily at the extreme positions or 

 elongations and moves fastest midway between. Smaller varia- 

 tions within the great period have been detected in the direction 

 of the needle. About the time of the maximum deflection 

 the magnet appears almost stationary for several years, but soon 

 a progressive motion commences, and, at first increasing, after- 

 wards diminishing its rate until the opposite stationary position 

 is reached and the motion reversed. Possibly this kind of a 

 "swing" may be repeated. Observation indicates that a com- 

 plete oscillation requires between 2-| and 3^ centuries, during 

 which time the magnet would swing twice through several de- 

 grees. Thus, at New York city the direction of the needle was 

 observed to be nearly invariable about 1685, pointing then nearly 

 9° to the west of north, it then moved easterly and reached its 



