Replacing floor joist

[dlgillen]

I have a house build in the 1930s but it looks like it was build with used (now we call it reclaimed) lumber. the floor joists look to be hand huned and may have been apart of a post and beam structure at one time. Regardless they seem to have rot and termite damage (thats being treated). The joists span 18 foot across the stone foundation and sit directly on the stone.
most joist are 8 1/2 to 9 inches wide and 2 1/2 to 3 1/2 thick. I plan to rip out the floor to gain access to the joists.
Once I have the floor out i plan to remove the old joists (due to rot and termites) and replace with 2x10x18 PT.
Questions:

1 should I double the 2x10s or will 1 2x10 across that span be good enough?
2. should I wrap the ends of the joist where they sit on the stone to help prevent future rot.
3. do I need to add extra support under the area where the load bearing wall is while I replace the joist?
4 Other suggestions about replacing joists

[Joel]

There are free joist span calculators, one is at awc.org I believe. That being said, 18 ft.is a long span for 2x10s. I'd double up. If you buy ground contact PT, you can rest directly on stone. Keep in mind the logistics of getting a full length joist in place. I'm not picturing where the load bearing wall is.

[dlgillen]

Thanks for the website. I have attached a floorplan. The 2 red lines are the load bearing walls. this house basically has 4 rooms with a single story addition in the back. The area between the red lines is a hallway and stairs. I had planned to put a bathroom in that area. When I took up the floor thats when I found the issue. I plan to rip out all the floor on the right side of the house (bedroom) and the hallway area and replace those joists first. Then work from the other red line and replace the left side of the floor (living room).