> The force that held the halves of the sphere together was the weight of the displaced atmosphere. More precisely: the force applied by the weight of the colu

> The force that held the halves of the sphere together was the weight of the displaced atmosphere. More precisely: the force applied by the weight of the column of air above the semi-spheres. Weight of the column of air -> air pressure -> force on semi-spheres. As parent noted, high up in the atmosphere that is a small force. At sea level, ~1 kg/cm^2. So: yes, it's gravity.