158 Trans. Acad. Sci. of St. Louis. 



had just prior to the explosion. When three sticks were 

 exploded the decrease in the intensity of the field was 

 still more marked. The needle was vibrating slightly 

 before the explosion. The amplitude increased from 

 two or three scale divisions to twenty-five. The decrease 

 in the intensity of the field was represented by a change 

 in the average reading of twelve scale divisions. 



It seems probable that the lines of force of the earth's 

 field are deflected around the space which is filled with 

 the gases liberated in the explosion, and that in the 

 space immediately adjoining, the field is thereby made 

 stronger, although the commotion of the air there also 

 tends to make the field weaker. The action is a differ- 

 ential one. At greater distances the main effect is due 

 to the agitation of the air, and here the field is made 

 weaker. There must be an intermediate position where 

 these two influences balance each other. The distance 

 of this position will depend upon the magnitude or vio- 

 lence of the explosion. 



In some of the preliminary work fifty sticks were 

 mounted in the air upon a raft of logs, which was an- 

 chored in the lake to the south of the station. Its dis- 

 tance from the observing station was about three thou- 

 sand feet. The effect of this explosion upon the needle 

 was inappreciable. It was at that time a very surprising 

 result. It now seems to be apparent, that an effect would 

 have been observed if the distance had been somewhat 

 less, or very much greater. 



Another source of disturbance of the magnetic field 

 was examined during the summer. The captains in com- 

 mand of the steamers plying between Mackinac Island 

 and Les Cheneaux Islands, were directed by Mr. Geo. T. 

 Arnold, president, to blow a long and loud blast on the 

 fog-horn, when at a point nearest to the station. There 

 were three opportunities on each day to make such obser- 

 vation. Plate XXXVII, Fig. 1, shows the location of the 

 steamer while the horn was in operation. This view 

 was taken from the roof of the porch in front of the 



