The amount of twisting motion of the building floor around the earth's axis varies with the latitude but is less than a Pole. The pendulum does not share this motion and therefore lags behind the floor -- and this can be seen. But the traveling motion of the pendulum eastward with the building is slower than the Equator -- but this you cannot see! That's why the pendulum lags behind only the twisting part of the floor's rotation and hence only loses part of the full circle to it in 24 hours.

some twisting and some travelling
the amount of twist being made
visible
by the pendulum,
but the amount of travel
cannot be seen.

 

earth spinning

Logical

degrees of twist
per day may be determined mathematically by this formula
n=360 sin phi
and can be demonstrated by using the pendulum
Phi = Latitude




by a Foucault Pendulum, man can demonstrate at any point on the Earth's surface (except on the equator) that the earth rotates--

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