166 Trans. Acad. Sci. of St. Louis. 



EXPLANATION OF ILLUSTRATIONS. 

 Plates XII-XVII. 



Plate XII. — 1, Electrograph with light from one window falling on the 

 left half of the plate. The right half is in shadow. Negative pole. — 2, 

 Electrograph with positive pole with plate exposed in a moderately dark 

 room. Discharge upon the large weight. 



Plate XIII. — 3, Same with small weight at a greater distance. — 4, Same 

 with the small weight still further removed. 



Plate XIV. — 5, Same with greater spark length. — 6, Same with still 

 greater spark length. 



Plate XV. — 7, Electrograph with negative pole, long spark length. —8, 

 Ball lightning discharges. — See diagram in text. Fig. 8 has been reversed 

 in the reproduction. 



Plate XVI. — 9, The same as Fig. 8. — 10, The same enlarged 75 diameters 

 and shown twice reversed. 



Plate XVII. — 11, Ordinary photograph — exposure 10 min. on a plate 

 previously fogged for two hours in X-rays, and developed as a positive in 

 the light. — 12, An ordinary photograph — exposure one minute. Developed 

 in the light. The original is also a positive. 



Issued May 16, 1900. 



