116 Trans. Acad. Sci. of St. Louis. 



practically disappeared before it reached tlie sun. Some- 

 times it was dispersed into smaller clouds, which were 

 more or less hazy in outline. Their effect might be ap- 

 preciable and persuasive, and yet more or less unsatis- 

 factory. 



The fact that it was impossible to predict at what 

 moment the desired conditions might present themselves, 

 and the necessity for having a record of the behavior 

 of the needle for a considerable time interval before 

 the sun entered a dense cloud, made it necessary to keep 

 the needle under constant observation, recording the 

 results during each minute of the day, sometimes at 

 lesser intervals, so far as this was possible. The only 

 interruption to this work during July and August was 

 from August 12th to 16th, during which interval a severe 

 attack of a painful illness made work of any kind im- 

 possible. 



The sun entered the well defined edge of the first 

 cloud above referred to at 10 :45 a. m. The needle had 

 been previously moving continuously in a direction such 

 as would be caused by a steady increase in the strength 

 of the field. When the sun entered the cloud, irregulari- 

 ties in the movement of the needle were observed. The 

 air on the border of a cloud shadow often gave evidence 

 of a disturbed condition. In this case the sun was in 

 the center of the cloud at about 11:10 a. m. This cloud 

 then covered the overhead sky down to about 45° from 

 the horizon. Below this cloud the sky was clear. The 

 sun reappeared at llh 12m 30 sec. At this time the scale 

 reading corresponded to the minimum shown in the dia- 

 gram, Plate XXIII. The needle at once reversed its 

 direction of movement. The reading at 11:40 or 11:50 

 was, as the diagram sliow^s, about what it would have 

 been if the cloud had not appeared. 



This cloud was soon afterwards broken up into smaller 

 clouds, and other smaller clouds appeared. From 12 :30 

 to 2 p. m. the sky was partly covered here and there 

 by smaller clouds, so that at the station, as at surround- 



