Z. A. Wagner has invented an apparatus for ascertaining the speed of vessels at sea, which appears to possess much. merit, and is certainly an excellent substitute for the old fashioned log and line. A brass blade about six inches long, is placed at the side of the keel, which, when not in use, is folded close against the keel, and presents no reSIstance to the water. By means of n rod passirJg through a tube fo the cabin or captain's state room, it connects with a dial plate. The apparatus is thrown into gear whenever the captain is desirous of knowing the rate at which the vessel is going, whichturns out the blade, so that the whole resistance of the water is thrown against it, and the exact speed is shown by a hand traversing the dial. The apparatus already made, to be affixed to the sailing yacht White Lily, goes as high as twelve knots, but it can be increased to any number necessary. The importance of know, ng to a fraction the rate at which a vessel is going, in order to guide the captain in his calculations, cannot he too highly appreciated. lExchange. The above invention is not new. For an llustrated description of the same thing see page 57, Vol. 6, Scientific American.