Why does a person who is standing soon become weary?
Because it requires a constant effort to keep the line of gravity over the feet, and this soon produces weariness.
Why can a person sit longer than he can stand?
Because the centre of gravity is lower, when he is sitting, than when he is standing; hence, it requires less strength to support the body in a sitting position.
Why can a person lie down longer than he can sit or stand?
Because the centre of gravity is nearest the earth when he is lying down, and it requires no effort to remain in that position.
How do we measure time?
We measure time by means of a pendulum, such as is used in a common clock.
How does the pendulum move?
The pendulum moves regularly to and fro, from the line of gravity.