In 1870-72, John Marius Wilson's Imperial Gazetteer of England and Wales described St Agnes like this:
AGNES (St.), One of the Scilly Islands, near the southern extremity of the group, separated on the NE from St. Mary's by St. Mary's Sound. It consists of two parts, St. Agnes-proper and the Gugh, divided from each other by high water of spring tides. The former measures about 1 mile by ½; the latter, about ¾ of a mile by ¼. Much of the land is fertile and well cultivated. St. Agnes-proper displays some interesting features, contains a church, and is crowned by a light house, 72 feet high, having a revolving light, with one minute flash. The Gugh has a curious rock pillar 9 feet long, and several stonecovered barrows. Pop., 200.
A Vision of Britain through Time includes a large library of local statistics for administrative units. For the best overall sense of how the area containing St Agnes has changed, please see our redistricted information for the modern district of Isles of Scilly. More detailed statistical data are available under Units and statistics, which includes both administrative units covering St Agnes and units named after it.
GB Historical GIS / University of Portsmouth, History of St Agnes, in Isles of Scilly and Cornwall | Map and description, A Vision of Britain through Time.
Date accessed: 14th October 2015
Click here for more detailed advice on finding places within A Vision of Britain through Time, and maybe some references to other places called "St Agnes".