Nipher — Vanations in the Earth's Magnetic Field. 123 



which were about a mile apart. Both found great dif- 

 ferences in the daily variation indoors and in open air, 

 and various possible causes for it are discussed. Barlow 

 concludes that it is probably due to the cause discovered 

 by Christie, that the intensity at any point in the field 

 of a bar magnet depends upon the temperature of that 

 magnet. He ascribes this difference to the different 

 temperature conditions of the control magnets. Never- 

 theless he is of the opinion that the quantity of daily 

 change in the earth's field depends in a greater degree 

 upon the intensity of solar light, than upon the mere 

 temperature of the day. Evidently his intuitive faculty 

 was of a high order. 



Barlow's conclusion did not carry with it any rational 

 explanation of causes, since at that time ionization of 

 the air, by sunlight, resulting in an increase of its per- 

 meability, was unknown. The work of Christie also 

 raised a doubt in the minds of others. Subsequent 

 writers seem to have found it necessary to say that the 

 daily variation and local magnetic storms had not been 

 satisfactorily explained. 



During the last fifteen years the present w^riter has 

 made various attempts to produce a local magnetic 

 storm in the earth's field by means of small amounts of 

 high power explosives. The results discussed in the 

 present paper seem to indicate beyond question that such 

 explosions are capable of producing magnetic disturb- 

 ances. The effect of the gust of wind shown in Plate 

 XXI was to produce an ether disturbance extending far 

 beyond and in advance of any air waves, or air disturb- 

 ance. 



Eecent experiments with dynamite on the grounds ad- 

 joining the buildings of Washington University have 

 not given conclusive results. It is difficult to eliminate 

 other disturbances, and it is not permissible to use as 

 large a quantity of the explosive as w^ill probably be 

 necessary. It is hoped that this work may be continued 

 at Hessel during the coming summer. 



