What is the circular well at the bottom?
Minus the plastic netting and steel mesh floor, which are NPS visitor safety equipment, this was originally a sand pit that was provided in case the weight cable of the clockwork mechanism parted causing the weight to free-fall down the tower. The sand pit was much more forgiving and flexible than the slate and marble floor that surrounds the pit. The weight set could also be lowered into the pit to facilitate maintenance and repairs such as cable repair or replacement. The well is about 4 ft deep.