Found bird egg; how can I hatch it?

I found a bird egg in my backyard last week and left it. It was still there this week. I felt guilty, wrapped it in a towel and put it in the mailbox to be away from the ground. Its very warm outside so that is where I left it. I shined a light through it to see if it was alive still and there is a dark bubble inside floating around and I can see some very light veins which leads me to assume the egg is viable but the mother is either dead or rejected it (obviously for a reason). I don't want to just discard the egg. I'm wondering if I can keep it. I would call the ASPA if it did hatch. I don't have a heat lamp, just the steamy Florida sun. I'm wondering what I can do for it to help it along and if anyone knows whether or not the egg is truly viable. Thanks very much for your help!