How does Newton's first law of motion work on sand clock?

 First Law of Motion : Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise.

How does Newton's first law of motion work on sand clock?

Answer: Gravitational force acts on sand clock. Without gravitation force sand clock will be always at rest therefore we can say gravitation force makes sand clock to flow sand from one bulb to another