118 Trans. Acad. Sci. of St. Louis. 



It may suffice to discuss briefly the record of July 19. 

 In doing this the weather maps kindly furnished by the 

 Weather Bureau at Washington were of material as- 

 sistance. On the afternoon of this day, the needle 

 showed more disturbance than on any other day. At 

 10 a. m. a rain cloud was observed in the southern hori- 

 zon. At 11:45 the needle began to vibrate, the aver- 

 age amplitude of vibration being about 20 scale divisions, 

 and sometimes reaching 35. The wind, which had been 

 from the northwest, had changed to the south. This 

 continued until about 12:10 p. m., when the needle be- 

 came less disturbed, and observations ceased until 12 :45 

 p. m. During this disturbance, it could be seen that a 

 rain w^as falling on the lake to the south. At 12:45 a 

 violent dash of rain began, which continued for ten min- 

 utes, and then continued as a milder rain until 1 :05. The 

 clouds were not continuous over the sky. The sun ap- 

 peared at intervals. 



The point of importance is that these vibrations began 

 and continued for an hour, while a rainstorm existed to 

 the south of the station, and before rain began to fall 

 at the station. During the remainder of the afternoon 

 periods of sunshine and rain came in alternation. Be- 

 tween 2 :37 and 3 :10 over half an inch of rain fell, from 

 what appeared to be a local cloud. The needle continued 

 to vibrate during this rain, and after it had ceased at the 

 station and w^hile its roar could be heard upon the lake 

 to the south. The wind was very mild during the entire 

 day, its velocity not at any time exceeding 8 or 10 

 miles per hour. The amplitude of the vibrations some- 

 times reached 40 scale divisions. The greatest ampli- 

 tude of the day was 49 scale divisions. The needle was 

 damped during this day. 



The data given on the weather map show that the 

 rain which visited the observing station extended from 

 Escambia, which is in northern Michigan, near 

 the north end of Lake Michigan,' to Alpena, which 

 is in southern Michigan, near the north end of Lake 



