A STUDY IX LOCOMOTION. 



321 



tion of this force does not give rise to a shock. If the cord which sus- 

 tains 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, hut 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 pecu- 



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. 



liar 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 car- 



riage-trace. 



VOL. XV. 21 



