386 Trans. Acad. Sci. of St. Louis. 
ing and a walk about 12 feet from the building was very 
dry, having been shielded by the building. During the 
forenoon the large masses and the injection pin for elec- 
trification of the air within the shield were connected with 
the lightning rod. Between 8:50 a. m. and 11:45 p. m. the 
scale reading decreased very slightly but with no vibra- 
tions. The temperature of the air around the shield in- 
creased by 1.°5 C. The injection pin was then withdrawn 
and the glass tube was covered with the metal cap. Vibra- 
tions of the suspended masses at once began. The scale 
reading varied through about three divisions of the scale. 
This amplitude had diminished to about two divisions of 
the scale at 1:20 p.m. The time of a complete vibration 
was about nine minutes. The frozen ground outside of 
the walk was then covered with a layer of water. Hight 
buckets of water were splashed over the ground around 
the lightning rod, covering the area over which the cop- 
per wires were spread, this wet area thus formed being 
finally connected with the wet ground beyond the walk. 
The scale reading abruptly diminished during the next 
three vibrations. The change in the average reading was 
about 80 per cent of the change formerly produced by 
the removal of the large masses to a position of no devia- 
tion in the position of the suspended masses. 
This determination of the deviation, due to the large 
masses, was obtained before electrification of the masses 
was begun. They were, however, then above the surface 
of the earth, and subject to the inductive action of the 
atmosphere. 
The results described in this paper indicate that the 
disturbances here discussed would be greatest if the 
work were done on a barren island, in a building above 
the level of the surrounding water. It is under such con- 
ditions that the phenomenon known as St. Elmo’s fire is 
commonly observed. On land each blade of grass and the 
leaves of trees have a function similar to that of the masts 
of ships provided with lightning protection. They are 
