120 Trans. Acad. Sci. of St. Louis. 



On two or three occasions phenomena were observed 

 which suggested that winds from the north, reaching the 

 station through a grove of trees, had a slightly different 

 effect from that of a wind from the lake. This subject 

 requires additional attention. 



Perhaps the most interesting phenomenon observed 

 during the summer was the continuous vibration of the 

 needle during a period varying from half an hour to 

 two hours preceding sunset. This was observed on 

 nearly every evening when the western sky was clear. 

 The vibrations were greatest when the day had been 

 clear, and the intensity of the field had reached a high 

 maximum. They did not occur when the afternoon sky 

 was covered with a dense cloud. The cloud shadow is 

 then joined to the earth's shadow\ They did not occur 

 in the morning, either before or after sunrise. Observa- 

 tions were sometimes begun as early as 2 o'clock a. m. 

 They were usually begun about an hour before sunrise. 



Plate XXIV, made from observations on August 31, 

 gives an illustration of these sunset disturbances. The 

 plate shows how the needle moved during the afternoon, 

 before the oscillations suddenly began. In the original 

 drawing, this part of the curve was drawn to a time 

 scale of 6 cm. per hour. For the period during which 

 the oscillations are represented, the time scale is 6 cm. 

 to about 3 minutes. One to and fro oscillation was drawn 

 to each half-centimeter. During this day the damping- 

 liquid had been removed. It will be observed that the 

 disturbing cause ceased at about 5 :45 and the needle 

 gradually came to rest. On this day two similar violent 

 disturbances occurred subsequently before sunset, which 

 are not represented in Plate XXIV. The greatest am- 

 plitude reached was about 250 scale divisions, or about 

 14° of arc. The extreme scale reading for each and 

 every vibration was read. 



In nearly all of the observations on this sunset dis- 

 turbance, the motion of the needle was restrained by 

 the damping fluid. The amplitude of the vibrations was 



