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 creased by l.°5 C. The injection pin was then withdrawn 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. Eight 

 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 



