Answer
Whether a team of horses pulls in the opposite direction with a force, or a sturdy wall pulls in the opposite direction with a force, the effect is the same.
Work Step by Step
Suppose the hemispheres were attached to a sturdy wall.
If one team of horses pulled on the hemispheres with a force $F'$, the hemispheres in turn would pull on the wall with the same force $F'$
By Newton's third law, the wall would pull on the hemispheres in the opposite direction with a force of magnitude $F'$
Whether a team of horses pulls in the opposite direction with a force of $F'$, or a sturdy wall pulls in the opposite direction with a force of $F'$, the effect is the same.
