tion of this force does not give rise to a shock. If the cord which sustains the weight of one hundred grammes is inextensible, and if that which hears the weight of ten grammes is the same, at the moment of the fall of the latter, you will hear a snap; a shock agitates the whole apparatus, but the weight of one hundred grammes is not raised.
Now suspend this weight of one hundred grammes to an India-rubber cord or an elastic spring, and repeat the experiment. You see, each time that the weight falls, that the hundred-gramme weight is raised to a certain extent. But this elevation is effected under peculiar
Fig. 1.—Apparatus to show that a vis viva directly applied to the displacement of a mass is lost in a shock, while the same force transmitted by an elastic medium may perform work.
conditions. At the moment when the weight falls and the cord is stretched, the balance inclines, stretching the elastic spring, but the mass of one hundred grammes does not yet move; it is only when this spring is stretched that the mass, obedient to the prolonged action of this elastic spring, begins to move and rises, representing a certain amount of work accomplished.
Thus the suppression of shock in traction economizes a certain part of the moving labor; it is then advantageous to give to the traces of a carriage a certain elasticity. One of the most simple methods consists in interposing between the trace and the carriage an elastic medium. Here are some of these elastic pieces, which I call tractors. One of the patterns has been made by M. Tatin; it is composed of a spring which is compressed by traction and deadens the shock. The other is formed of a similar spring placed in the very inside of the carriage-trace.
