E. H. Hall— Rotation of Equipotential Lines, ete. 117 
Discina Acadica Hartt is now referred to the genus Stenotheca. 
When examining specimens of Stenotheca rugosa Hall, sp., from 
the Georgian horizon at Troy, New York, in the collection of 
Mr. S. W. Ford, I noticed’ compressed specimens that are 
almost identical in appearance with the type of Professor 
_ Hartt’s species now before me. Mr. R. P. Whitfield* called 
attention to this resemblance, believing that D. Acadica was the 
Impression of a univalve shell of the genus Paleacmea or Sten- 
otheca. 
DESCRIPTION OF FIGURES. 
Fig. 1. Plan of the interior of the dorsal valve of Obolella chromatica as deter- 
mined by Mr. Billings. Fig. 2. Ditto of the ventral. Fig. 3. Outline of the interior 
valve of L. sagittatis, after Davidson. Fig. 6. Interior of the dorsal valve of Z. 
transversa, as shown in a flattened specimen. Fig. 7. Cast of the interior of the 
dorsal valve of an uncompressed example of Z. transversa. Fig. 8. Cast of the 
Interior of the dorsal valve of L. sagittalis, after Davidson. 
ART XIX.—On the Rotation of the Hquipotential Lines of an 
Electric Current by Magnetic Action; by E. H. Hatt, Instruc- 
tor in Physics at Harvard College. 
In this article the results will be given of experiments made 
during the month of Au ust, 1883, and at intervals since, in 
the Physical Laboratory of Harvard College. The substances 
which have been chiefly examined are copper, zinc, certain of 
their alloys, and iron and steel. Some mention will be made 
of gold, cobalt, nickel, bismuth and antimony. In most 
®ases when possible the metal was used in the form of a thin 
Strip about 1-1 cm. wide and about 8 em. long between the two 
Pieces of brass B, B (fig. 1), which soldered to the ends of the 
Strip served as electrodes for the entrance and escape of the 
Farts shown in the figure being imbedded in and covered by 
this cement, which was so hard and stiff as to be quite brittle 
at the ordinary temperature of the air. : 
The plate of glass bearing the strip of metal so imbedded 
Was, when about to be tested, placed with B, B, vertical in the 
2. This tank T T containing the plate of glass with the metal 
* Bull. Amer. Mus. Nat. Hist., vol. i, p. 140. 
Ax. Jour, Sci.—Tairp Series, Vou. XXIX, No. 170.—Fxss., 1885. 
8 
