112 Trans. Acad. Sci. of St. Louis. 



amount by closing the circuit containing the compensat- 

 ing coils, and varying the current by means of a proper 

 resistance (carbon plates with a compression screw). 



It was of course found that in general the intensity H 

 of the horizontal component of the earth's field, increases 

 during the day, reaching a maximum late in the after- 

 noon. The numerical value of H is, however, greater on 

 clear days than on cloudy days. On days which are clear 

 in the forenoon and cloudy in the afternoon, the maximum 

 may occur in the middle of the day, or before noon. Dur- 

 ing days when the air is quiet, the needle is more quiet 

 during cloudy or clear days, than when the sky is cov- 

 ered with small clouds with blue sky between. 



On days when the wind blows in gusts at intervals of 

 one or two minutes the needle is more unsteady in its 

 movements, than on quiet days, or on days when the 

 wind is more uniform. 



On days when gusts of wind are frequent, it is impos- 

 sible to identify any particular wind-gust with any par- 

 ticular disturbance of the needle. The reason for this 

 appears to be explained by an observation made on 

 July 14. During the forenoon of that day the wind was 

 very mild from the west. Shortly before 1 o'clock the 

 wind suddenly changed to the south, while it continued 

 at the rate of 3 to 4 miles per hour. At 1 :10 p. m. the 

 needle began to vibrate to and fro. The scale reading at 

 each extreme position was recorded. This was continued 

 for nine minutes, when a blast of wind came in from the 

 lake to the south. It overturned a sail boat lying at a 

 dock about 200 ft. distant, the sails of which had been 

 raised in order to dry them. It was by far the most 

 violent wind of the summer. About eight minutes later 

 the wind had practically ceased, and the vibrations of 

 the needle had also ceased. The scale reading had been 

 under constant observation before the gust of wind be- 

 gan. The reading was recorded each minute, and even 

 more frequently during times of mild disturbance, when 

 slow to and fro movements made this necessary. When 



