Thanks to everyone for their comments.

Here's a link to a website that shows a picture of a similar barn to the one I'm looking at. The top right hand picture of the "1840's Moffat Barn" shows a large horizontal timber that is conciderable thicker in the middle and tapers to the end. I believe this is a "Swing Beam"

http://www.historiclumber.ca/salvage2.html

In fact the rafter and purlin system is similar to this barn as well.

I'll try to get some additional details to share. Appreciate the help.