Nipher — Variations in the Earth's Magnetic Field. 117 



ing points, the sun was visible at and during frequent 

 intervals. One of these small clouds unexpectedly gave 

 rise to the dash of rain before discussed. The general 

 effect of these clouds is shown in the general drop in the 

 curve between 12:30 and 1:30 p. m. 



The sun entered another dense and sharply denned 

 cloud surrounded by clear sky, at 4 :20 p. m. of this same 

 day, after the daily maximum had been passed. It 

 emerged from this cloud at 4:28 p. m. While the sun 

 was hidden by this cloud, the intensity of the 

 magnetic field diminished as in the other case, as 

 is shown by the drop in the curve. When the sun 

 reappeared the intensity at once began to increase. At 

 4:37 the reading was that corresponding to the general 

 trend of the curve during that afternoon. The time of 

 entering and leaving the cloud is in both cases indicated 

 on the diagram by arrows. 



It would thus appear that cloud shadows during the 

 day have the same effect upon the earth's magnetic 

 field that the earth's shadow has at night. The lines of 

 the field sway around them. They sway above the clouds 

 into the sunlight. The horizontal component would thus 

 be diminished below the clouds. 1 



It is impossible to present here the full evidence ob- 

 tained, which to me seems to establish beyond all question 

 the conclusion that local variations in the earth's mag- 

 netic field are determined wholly by local weather con- 

 ditions. It is contained in 300 pages of closely written 

 notes on pages 8 inches square. While it might at first 

 seem that the greater part of this record was of no im- 

 portance, it does establish the general conclusion that 

 when local conditions were uniform, whatever they might 

 be, the magnetic needle showed no marked disturbance 

 of an abrupt character, such as we have in these vibra- 

 tions. 



1 Observations of the character here discussed should be made during 

 solar eclipses. 



