effects of these rotations on the pendulum: just as at page 111, the effects of the two components of a force are separately considered.
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Fig. 68.
If, then, the earth rotated about OP alone, P would be one of its poles; and therefore, as has been already explained, to a spectator near P the plane of vibration would appear to revolve in the same time as the earth would revolve about OP alone, but in the opposite direction. Again, if the earth revolved about OR alone, P would be situated on the Equator; and a spectator would not observe any change in the plane of vibration. Thus at every place not situated on the Equator, the plane of vibration will appear to change its position; and such change will be entirely due to the component rotation about OP.
It is not difficult to submit these conclusions to an experimental test, and this has actually been done at many places. Care must be taken not to give the sphere any lateral motion at the moment
