It is our melancholy duty to record the most fearful and fatal steam-boat explosion which has ever taken place on the waters of the Chesapeake. The Medora was just completed, and, preparatory to being turned out of the hands of the machinist, a number of persons were invited to go in her on an experimental trip. The day being pleasant, it is supposed that probably 100 or more were on board, including some of the directors of the steam-boat company and their friends, and a number of the hands engaged in finishing the vessel, putting in the machinery, &c., and otherwise connected with the construction or sailing of the boat.