Nipher — Vanatioiis in the Earth's Magnetic Field. 115 



plate representing the hour of the day. The needle was 

 in a more disturbed condition after the vibrations had 

 ceased than it had been before. Evidently the rain had 

 some effect upon the magnetic field at the station when 

 it was falling through air to the south of the station, 

 and had ceased at the station. The needle was damped 

 during this day, so that summation effects were impos- 

 sible, nevertheless in one of these oscillations the needle 

 vibrated through an arc of over 5°. 



This rain occurred on August 26. A diagram repre- 

 senting the movement of the needle between 10 a. m. 

 and sunset, is presented in Plate XXIII. The vibrations 

 due to the rain occurred at the close of the first gap in 

 that curve. They cannot be properly represented here 

 with the time-scale used in this plate. They are replaced 

 by a straight line. The second gap in the diagram rep- 

 resents the time required to finish the noon-day meal, 

 which had been interrupted by the rain. The part of the 

 diagram of Plate XXII, after the hour of 1:05, is the 

 part which in Plate XXIII lies between the two gaps. 



Plate XXIII also represents the effect upon the hori- 

 zontal component of the earth's field, of tvv-o large dense 

 and sharply defined clouds passing over the sun. Their 

 effect is shown at points corresponding to 11:10 a. m. 

 and 4 :28 p. m. During seven weeks the needle had been 

 under constant observation, from sunrise to sunset, in 

 order to secure the results which were here obtained, 

 On many days evidence of cloud effects were observed 

 which seemed conclusive, but usually the edges of the 

 cloud would not be sharply defined. In some cases the 

 edges would be more or less transparent, in some cases 

 small clouds of irregular form would surround the larger 

 cloud. It often happened that large dense clouds ap- 

 peared to be approaching the sun, and the needle gave 

 results which were wholly different from those which 

 had been expected. On going out of the tent it would 

 be found that the cloud had behaved in a wholly different 

 manner from what had been expected. In some cases it 



