io6 



NATURE 



[Dec. II, 187^ 



For example : — The interval of time between the de- 

 parture and return of the wave produced by the concussion 

 is j of a second ; the double journey is represented by 



1 10 m., and the distance of the obstacle is -— 



330 m 



3 



= 55 metres. 



The times of the departure and of the return of the 

 wave are graphically registered on a chronograph, by the 

 interruption of an electric circuit obtained by the motion 

 of a-membrane of caoutchouc placed at the extremity of 

 the tube. 



It is known that an electric current magnetises a horse- 

 shoe magnet. The magnetisation of the magnet com- 

 municates to a palette placed above the poles, an attrac- 

 tion which ceases as soon as the current is broken. 

 Without entering into further explanation of this well- 

 known arrangement, which is the basis of nearly all 

 telegraphic apparatus, it will be granted that with 

 conveniently placed conductors it will be possible 



to make the armature of the magnet move like the 

 elastic membrane ; in other words, if the membrane 

 is raised 2, 3, 4 times in a second, the armature will be 

 connected 2, 3, 4 times, and the durations as well as 

 the intervals of the contacts will be identical in the two 

 apparatus. 



Let us return to the chronograph. The time is 

 marked by it, and is recorded by means of electro- 

 magnets. The oscillations of a seconds pendulum 

 are repeated electrically and registered on a line, No. 2 

 in Fig. 4, which is described by a point fixed to the 

 electro-magnet, upon a smoke-blackened cylinder, to 

 which is given a movement of continuous rotation. 

 The electro-magnet whose point describes line No. 

 2, is moveable on a carriage that advances along 

 the cyhnder in the same time as the latter takes 

 to turn. The carriage bears two other electro-mag- 

 nets : one corresponds to a sub-divisor of the time 

 which gives fractions less than a second. It is this which 



Fig. 4. — Chruiiograpli 



traces line No. i, representing by its tracings sub-divi- 

 sions equal to -f^ird of a second ; this division into frac- 

 tions corresponds to the oscillations of the palette of an 

 electric iycmbleiir, a contrivance in which the interrup- 

 tions and re-establishment of the current take place at 

 the rate of 33 per second in the model here represented. 



The third electro-magnet, in connection with the mem- 

 brane of caoutchouc, corresponds to the movement of the 

 wave in the tube ; it furnishes line No. 3 in the figure. 

 It may be remarked that the same wave undergoes many 

 successive reflexions. 



It will be easily seen from the diagram how the result 

 sought can be obtained from the experiment. Suppose 

 the obstacle to be placed at 62 metres ; the interval be- 

 tween two successive marks of the membrane is about 

 12 sub-divisions. A comparison of lines i and 2 shows 

 that there are 33 sub-divisions in one second ; the indica- 

 tions of line 3 then are equal to W of a second. The 



double distance represents \r^ x 330 m, and the simple 

 length given by the experiment is thus about J X iS x 

 330 = 60 metres, the result sought to within 2 metres.' 



Fig. 4 shows the method adopted for producing the 

 wave. On the left T is the tube in which a small pistol 

 V is placed to produce the detonation which gives rise to 

 the wave. On the table in the centre of the figure is the 

 chronograph ; M is the clock-work which turns the regis- 

 tering cylinder, on the surface of which are traced the; 

 lines I, 2, 3 ; S is the carriage bearing the three electro- 

 magnets, each of which traces its line. The electro- 

 magnet on the right, line i, is the tn'inhlciir,\vi connection 

 with the pile P P". The middle electro- magnet, line 2, 

 is connected with the seconds pendulum R. Finally, the 

 electro-magnet on the left, line 3, communicates electri- 

 cally with tlie caoutchouc membrane that surmounts the 

 tube T, and exactly fits the opening, on which it is 

 stretched like a drum-skin. 



