Our readers have often seen grocers' clerks or employes of business houses break the string with which they had tied up a package, by seizing it with the hands, bringing the latter close together, and then suddenly separating them with a quick movement. If it be thought that this quick motion is sufficient, let any one try it, and he will merely cut his hands without breaking the string, provided the latter has some little strength. In order to succeed, the cord must be arranged in a certain manner, as we shall explain. [Illustration: MODE OF BREAKING A CORD WITH THE HANDS.] The cord to be broken is placed upon the left hand, and one of its ends is passed over the other in such a way as to form a cross, and the end forming the shorter part of the cross is wound around the fingers (it should be left long enough to make several turns). The other end is then turned back and wound around the right hand, so as to leave a space of about eighteen inches between the latter and the left hand. If these directions are properly followed, the string should have the form of a Y in the middle of the hand, as shown in the lower figure of the accompanying engraving. It is only necessary after this to close the hand, after seeing that the Y is very taut, and to seize the cord with the other hand, as shown in the upper figure. This done, the two hands are brought together and then suddenly separated so as to give a quick pull on the point of junction of the Y-shaped branches, which form a true knife. It will be readily seen that as the cord is broken suddenly the shock does not have time to transmit itself to the hands. This is an interesting demonstration of the principle of inertia. * * * * * AN AQUATIC VELOCIPEDE FOR DUCK HUNTING. The curious apparatus that we represent in Fig. 1, from an old English engraving of 1823, is an aquatic velocipede which was utilized with success during the entire winter of 1822. An amateur employed it for hunting ducks upon the numerous streams of Lincolnshire, and, as it appears, obtained very good results from it. The device is very ingenious. It consists of three floats of from 1,800 to 2,000 cubic inches capacity, made of copper or tin plate. These are full of air, and must be perfectly tight. They are held together by arched iron rods, as shown in the cut, so as to form the three angles of an isosceles triangle. These rods are provided in the center with a saddle for the velocipedist to sit upon. The apparatus floats upon the water and sustains the hunter, whose feet are provided with quite short paddles, by means of which he navigates, and steers himself. [Illustration: FIG. 1.--AN AQUATIC VELOCIPEDE OF 1822.]