442 



NATURE 



[Marc/i 13, 1879 



a progress supposed uniform ; we see, in fact, that the 

 lines of progress are all straight, joining to each other 

 the two points which express the place and time of de- 

 parture, the place and time of arrival. It does not then 

 take into account the real movement of the train, which 

 is accelerated or retarded under a great number of influ- 

 ences. The problem which we seek to solve, that of a 

 graphic expression of the real rate of a vehicle, supposes 

 that the carriage itself traces the curve of the roads tra- 

 versed, in function of time. By means of the apparatus 

 which I present to you, and which I call Odograph (Fig. 

 =6), a waggon or any kind of carriage traces the curve of its 

 movement with all its variations. 



This apparatus, based on the same principle as the 

 Poncelet and Morin machine, is composed of a tracing 

 •style which moves parallel to the generatrix of a revolv- 

 ing cylinder covered with paper. The movement of the 

 style follows all the phases of that of the carriage, but 



on a very reduced scale, in order that the tracing of a 

 distance of several myriametres may be contained in the 

 dimensions of a sheet of paper. As to the movement or 

 the cylinder, it is uniform, and commanded by clockwork 

 placed in the interior. In order that the movement of 

 the style may be proportional to that of the vehicle, 

 things have been so arranged that each turn of the wheel 

 causes the style to advance by a small quantity, always 

 the same. But as a turn of the wheel always corresponds 

 to the same distance accomplished, the faster the vehicle 

 travels the more turns will the wheel make in a given 

 time, and the more movements of progression will the 

 style undergo. This solidarity between the movements 

 of the wheel and those of the style is obtained by means 

 of a small excentric placed on the vane. At each turn 

 there is produced a pufF of air, which, by a trans nitting 

 tube, causes a tooth of a wheel of the apparatus to 

 escape, and the style to advance by a small quantity. 



HeuretsO^ 



Ih 



10- 



20- 



Fig. 7. — Tracings of the Odograph. a, rapid coach with stoppage ; b, slow coach ; c, gas meter, frequency of turns of the wheel ; d, curve of the 



turns of a clock wheel-work with fly. 



Similar effects may be obtained by means of electro- 

 magnetic apparatus. Thus the swifter the vehicle goes 

 the more rapidly will the line traced ascend ; the com- 

 parative slope of various elements of the tracing will 

 •express the variations of rate, as seen in Fig. 7. 



If we wish to learn the absolute value of time and dis- 

 tance, it is sufficient to know that each minute corre- 

 sponds to a millimetre counted horizontally on the paper, 

 and that each kilometre corresponds to a certain number 

 of millimetres traversed by the style in the vertical direc- 

 tion. The course of the style, which corresponds to a 

 kilometre, ought to be experimentally determined for 

 each vehicle, for the perimeter of wheels is not always 

 the same. But it is clear that, if from each kilometre- 

 stone to another we obtain five millimetres, for example, 

 for the course of the style, this length will always be 

 found to be traversed each kilometre by the same vehicle. 

 Our apparatus is then a measurer of distances, and dis- 

 penses with the necessity of attending to the existence 

 of kilometre-stones ; it enables the distance traversed on 



any road whatever to be estimated, and evea when there 

 is no beaten track. Thus in a journey of discovery we 

 may measure the distance traversed by a cart. To 

 remain in the conditions of ordinary life, have we not 

 sometimes, in the country, a choice of two or three roads 

 to go from one place to another ? To know which is the 

 shortest, we appeal to the watch, as if the least duration 

 of a walk corresponded to the least distance. The odo- 

 graph will give in this respect very precise information. 



There is again a great number of questions which we 

 ask daily without being able to solve them. Does such 

 a draught horse go quicker than such another? Does this 

 trot better to-day than yesterday? By increasing the 

 ration of oats do we increase speed ? Compare the slope 

 of two curves of rates, and you will have the reply to all 

 these questions without being obliged to make special 

 experiments on a measured road, watch in hand. 



It is not only to the speed of vehicles that the register- 

 ing apparatus applies; it traces, though with less pre- 

 cision, the rate of progress of men and animals. We 



