A LUCKY FIND. – The other day a man named ROBINSON, engaged at the steam mills of MESSRS. SEATREE, at the top of Castlegate, found amongst some riddlings an American 20 gold dollar,, which is supposed to have come over sea in a bag of wheat, either from the former place or from California from which countries MESSRS. SEATREE import largely. The dollar, previous to being discovered must have passed twice through the seive, and been afterwards thrown amongst the refuse in the yard.