214 On the Pendulum Experiment. 
its plane has therefore been accounted for. It is now evident 
‘that the Foucault problem admits of some extension. 
1. Let us suppose a continuous hollow ring filled with water 
or some other fluid, the heavier the better, and suspended sym- 
metrically in a vertical plane by a proper cord. Let the fluid be 
kept in continual cireulation. by means of a lamp attached to one 
side of the ring.. Then will.the ring revolve precisely like the 
plane of the pendulum and for the same reason :—because the 
suspended mass is compelled to be equally on both sides of the 
vertical axis or suspending cord. The rotation of the ring how- 
ever must be somewhat retarded by the material. of the ring, 
lamp, &c., which do not partake of the motion of the fluid. 
2. Let a similar ring be placed symmetrically upon a small in- 
flexible axis parallel to the axis of the earth, and let the fluid be 
kept in circulation as above. The ring will rotate no longer 
with the pendulum of our latitude, but with the pendulum at the 
pole of the earth, or once in twenty-four hours. 
Again let the axis of the ring receive any direction, the ve- 
locity of rotation will be that of the pendulum at the place 
whose vertical line is parallel to that direction, or equally inclined 
with it to the axis of the earth. 
really, by a vertical cord or axis, or by an axis inclined, as above, 
around which it is free to move. The result will be the same as 
in 1 and : 
It would seem therefore that the Foucault experiment for all 
latitudes may be illustrated at any place, but it must be admitted 
that there are many practical difficulties to overcome—particularly 
when the principal axis is not vertical. These suggestions are 
thrown out for those who have the leisure, the means and the 
>ome have supposed that any horizontal plate or bar, sus 
pended bya cord, would: continue to revolve like the plane of 
the pendulum if the proper motion could once be communicat 
—forgetting that the retarding force of friction, which is always 
acting, must soon destroy the motion communicated by al im- 
pulse. If all friction could be destroyed the bar would indeed 
maintain any rate of motion once communicated to it. 
It has been suggested that the proper motion would be com> 
municated to a suspended bar if its two ends were suddenly re 
versed and then allowed to move with the velocities previously 
acquired. The motion thus communicated would be double the 
one desired. The proper motion is communicated by the pendu- 
lum or by the means described above. To keep that motion UP 
