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POPULAR SCIENCE REVIEW. 
instruments lay unheeded in a dark and solitary corner. Had this ill-used 
and ill- appreciated nucleus of an English Conservatoire des Ai'ts et Metiers 
continued longer, this and many other historical instruments of inestimable 
value, would in time have received due attention. 
The disc was furnished with a small driving pinion, and being removed 
from its delicate suspension, was dropped on the top of a vertical mill or 
driving apparatus of multiplying wheels, terminating in a handle. It was 
thus whirled up to a high velocity, and then gently lifted off the driving 
apparatus into its position of suspension. The rotation thus imparted could 
be sustained for above an hour, and even more in a fair vacuum. But from 
the first it was essentially moribund ; and it is only recently that an attempt 
has been successfully made to render it self-supporting. The machine con- 
sists essentially in a wheel of very heavy rim, freely supported on screw 
points. If the frame in which the wheel turns be itself suspended in a 
second frame, so that the axis is unconstrained, the position of original rota- 
tion will be maintained independently of the motion of the earth. The efiect 
of gravity in tending to keep the disc in a fixed position, relative to the 
earth’s surface, is in fact annihilated. A hand fixed to the frame in which 
the wheel revolves will move over a divided circle attached to the base of 
the instrument with an angular velocity equal to 360° multiplied by tho 
sine of the latitude of observation. 
It has been ingeniously suggested to apply this motive power by means of 
electricity. Clockwork and weights or springs are evidently inadmissible. 
Mr. G. M. Hopkins has overcome constructive difficulties, and rendered the 
instrument of great practical value. 
The disc carries a soft-iron armature close to the face of a small electro- 
magnet included in the frame. An insulated contact-spring is arranged to 
touch twice during each rotation, and to cause attraction of the armature. 
One battery connection is made through the supporting pivot, the other by 
means of a wire dipping in a ring-shaped mercury cup. Two or three 
Bunsen’s cells are sufficient to maintain rotation at the necessary velocity. 
Dilatation by Electricity. — Mons. Duter drew the attention of the Academie 
des Sciences to the fact that when a Leyden jar is formed of a long tube, 
encased in tinfoil, and filled with water or a saline solution, on its receiving an 
electrical charge the level of the contained fluid sinks appreciably ; on dis- 
charge it instantly resumes its former level. The natural conclusion is, that 
the glass of the jar dilates. This phenomenon occurs whatever be the nature 
of the armatures, foil, water, or mercury. If the jar thus formed be placed 
in an outer tube containing fluid, and acting as an outer coating, the latter 
ascends, while the former sinks. On discharge they both return to their 
former level. It is, therefore, obvious that internal capacity and external 
volume increase during the charge of a Leyden jar. The phenomenon cannot 
be due (1) to rise of temperature, as it suddenly disappears on discharge ; 
nor (2) to electric pressure, which, being the same on the two faces of the 
dielectric, would tend to diminish its volume ; nor (3) to the superior 
attraction of the electrified jar for the liquid, since it ought, in this case, to 
sink in the outer jar also*, nor (4) to differences of the positive and negative 
armatures, for if the communications be reversed the direction remains 
unchanged. The cause seems to be unexplained. 
