Motion of the Medium on the Velocity of Light. 381 



wooden support entirely disconnected from the refractometer 

 which was mounted on brick piers. 



Fig. 



Explanation of Figures. 

 1. — "Vertical section through tubes. Fig. 2. — Plan of tubes 



Fig. 3 — 

 Fig. 4. — Gauge for ve- 



One end of tubes, showing glass plate inclined to axis, 

 loeity at different points. Fig. 5. — Plan of refractometer. 



The flow of water was obtained by filling a tank four feet in 

 diameter and three feet high, placed in the attic, about 23 

 meters above the apparatus, with which it was connected by a 

 three inch pipe. The latter branched into two parts, and each 



