Nipher — Variations in the Earth's Magnetic Field. 121 



then in general over an angle not exceeding 50 scale di- 

 visions or 2.8 degrees. The time of vibration was prac- 

 tically the same whether the damping fluid was used or 

 not. It was not uniform in either case. The vibration 

 of the damped needle frequently continued without ces- 

 sation for an hour or more. At this hour of the evening- 

 there was usually no wind at the station. 



It is evident that along what may be called the sun- 

 set meridian, there will generally be places where cloud 

 shadows are joined to the earth's shadow. Since only 

 the horizontal component of the earth's field of force is 

 effective in action upon the needle, we may consider the 

 conditions which would exist in a field of force in which 

 the lines are horizontal. Where the clouds occur, these 

 lines tend to sway above the clouds into the sunlight. 

 If we consider these lines to behave like elastic threads, 

 they are elongated by this distortion. They snap asun- 

 der and disappear as they are thus distorted and forced 

 towards the approaching shadow of the earth, the field 

 diminishing in strength in a rhythmical way. This state- 

 ment must be considered as figurative in character, but 

 it is in a certain sense descriptive of the observed phe- 

 nomena. It is with some surprise that I find that this 

 sunset disturbance has not been observed at stations 

 where continuous records are made. This conclusion 

 obtained from an examination recently made of gov- 

 ernment publications has been confirmed by information 

 just received from the chief of the U. S. Coast and Geo- 

 detic Survey. Evidently the subject deserves additional 

 attention. 



It will of course be understood that the fact that wind- 

 gusts a few miles distant are found to affect a magnetic 

 needle, is in harmony with the well-known fact that solar 

 disturbances also affect it. 



The observation of Young at Sherman, Idaho, on 

 August 3, 1872, showed that a solar outburst produces 

 electromagnetic waves, which travel with the velocity of 



