Webster-Ashburton Treaty

The treaty of 1842 "agreed on a division of disputed territr, giving 7,015 square miles to the United States and 5,012 to Great Britain" (history.state.gov/.../webster-treaty).  So for the U.S. Census we are talking about the 1840 and 1850 census.  The 1840 U.S. census is available at: https://www.familysearch.org/search/catalog/745492?availability=Family%20History%20Library registration needed, but it's free.  The Census is divided into counties.  The 1850 Maine Census is at: https://www.familysearch.org/search/catalog/744482?availability=Family%20History%20Library with a more detailed boundary definitions.

Laurie,  Besides my earlier reply, you might if you haven't seen this find a copy of William Thorndale and William Dollarhide's 'Map Guide to the U.S. Federal Censuses 1790-1920"...copyright 1987, that shows the maps for Maine 1840 and 1850 and the changing boundary line between Canada and the U.S.