In what direction do all objects fall through the air?
They all fall towards the centre of the earth.
Which way then do we call “down”?
Down is always towards the centre of the earth.
Which way do we call “up”?
Up is always away from the centre of the earth.
Where must all plumb lines or perpendicular lines point?
They must all point with one end towards the centre of the earth.
When the earth turns over, can any substance fall away from its surface?
No; all things are drawn to the earth with so much force that they cannot fall away.