158 Trans. Acad. Sci. of St. Lows. 
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 
