The steamer Fanny Fern, bound to St. Louis from Pittsburg, exploded her boiler eighteen miles below Cincinnati on the 28th ult. Fifteen persons are reported to have lost their lives, and thirty were severely injured ; the boat was set on fire by the explosion, and burned to the water's edge. This is the most severe accident of this character, we believe, which has taken place on any western river since the new law for protecting life in steamers went into force. We are afraid that the western steamboat inspectors are becoming somewhat careless ; and we have good reasons for believing that is also the case with those in the eastern districts. We exhort these officers to look well to their regulations, or they may soon find their situations too hot for them.