H. Amory—Application of the Horizontal Pendulum. 21 
IL, Meramorpuic CHANGE with crystallization of 
(a.) Octahedrons of magnetite and the discordai crystals. 
(0.) Garnets. 
Ill. PsevpoMorPHIC CHANGE. Chlorite after the original rock 
and after garnet, but preserving the magnetite and dis- 
coidal crystals intact. ; 
Arr. IV.—Brief Contributions from the Physical Laboratory or 
Harvard College. No.18.—An Application of’ the Horizontai 
Pendulum ; by Harcourt Amory. 
difficult to make. By the use of the horizontal pen ulum, 
however, the mutual action of the currents can readily 
shown. The apparatus is arranged as in the accompanying 
figure. The wire which forms the upper support of the pendu- 
lum is connected with one pole of a battery, and is then | 
along the horizontal bar of the pendulum, best made of glass, 
and is bent in the form of a parallelogram at the extremity of 
the bar. The wire is then led back to form the lower support 
of the pendulum, and is then connected with the other pole of 
the battery through a fixed coil of wire placed in the neighbor- 
hood of the end of the pendulum. 
The current first passes to the upper suspending wire, around 
the oe at the extremity of the pendulum, back 
through the lower supporting wire, through the outside coil, 
and returns to the battery. By turning the outside coil upon a 
re —— axis, pane Jaws of attraction or repulsion a 
“ents can be shown. The apparatus is peculiarly wel 
adapted to show the action of solenoids upon cs aoe | 
