A Jack Is No Substitute for a Foundation
Old houses tend to sag after a while. The correct fix involves jacking the floors back into place and then adding structural beams to keep them there. Some do-it-yourselfers jack the floor back in place and stop there.
Years go by and nothing changes. One end of a beam still rests on the sill. That old, reliable jack still holds up the other end. It is now a permanent fixture. The floor is firm and level. Just don’t let your insurance agent, mortgage holder or city engineer see it.
The only option here is to finish the job correctly. Your home would be a safer place to live and easier to sell when the day comes.