Hi Chris,This is it - a sliding seat bicycle:This frightening machine was invented by one Louis S Burbank from Worcester, Massachusetts in 1900. Looks as if it could go fast - can't see any brakes, though.
found my solution to boaty activity without risking the water: http://www.rowinghistory.net/Patents/642,544.pdf. A stumbled find when looking for Barrett rowlocks for the Rangeley! One of those sites you just get sucked into - another hour wasted when I should have been clearing the shed!