46 MOVEMENT 



must traverse the width of the paper in one hour. 

 Besides this, as the strip of paper is drawn through 

 the rollers, it passes beneath a comb, which registers 

 equidistant lines along the length of the paper; 

 the distance between two teeth corresponds to a 

 duration of ten minutes. This greatly facilitates the 

 reading of the distance traversed in a particular time. 

 Thus during every hour the needle traces a section of 

 a curve analogous to those in Fig. 27, and which are 

 distinguished in order by the letters A, B, C, — F, 

 because the needle takes exactly one hour to cross the 

 width of the paper. As soon as the first curve has 

 been registered another one is commenced, because a 

 second needle in turn begins to register, and at the 

 third hour another needle, and so on indefinitely during 

 the whole of the journey.* When the strip of paper 

 ceases to advance, during periods of rest, the clock 

 nevertheless continues to move the needle, and the 

 latter describes a straight line at right angles to the 

 long axis of the paper. This way of registering move- 

 ment is identical with that which was designed by 

 Ibry. We believe that odography could, with a few 

 special modifications, be applied for registering the 

 progress of a railway train. A novel arrangement of 

 pneumatic tubes can transmit each revolution of the 

 wheels of the engine to the mechanism of the 

 cylindrical rollers, and thus an odograph can be 

 placed in each compartment, and supply continuous 

 information of the progress of the train. Thanks to 

 the kindness of M. Millet, chief engineer of the loco- 

 motive department on the Southern Railway (Chemin 

 de fer du Midi), we were able to try the effect of the 

 odograph on express trains running from Dax to 

 Bordeaux, and vice versa. Fig. 33 shows in column A 



* The needles are driven by an endless steel band controlled by 

 the clock. 



