598 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



facsimile of the written message, or if Deed be, of drawings, charts, 

 plans, or portraits. It is thus a veritable autograph that the receiver 

 of the message gets from the sender, so as to have in his hands, if 

 required, an authentic document. What could be more extraordinary 

 at first starting than the solution of such a problem ? but we shall 

 see that nothing can be easier than to understand the means by which 

 this solution is realised. 



Suppose we have fitted in the two stations, the sending and receiv- 

 ing, two plates of copper, M K (Fig. 390), communicating with the 

 earth at T. On the plate M of the departure station is placed a sheet 

 of metallized paper. On this sheet the message is written by the 

 sender himself in greasy insulating ink. At the other station on the 

 plate E, is placed a sheet of paper, previously soaked in yellow ferro- 

 cyanide of potassium. Two iron styles, s, s', are connected with the 



FIG. 390. Principle of Caselli's autographic telegraph. 



battery and the line-wire, and move synchronously, describing with the 

 same velocity very close parallel lines on the two sheets of paper. 

 We shall see further on how these styles are moved, and how their 

 motion is regulated by pendulums which oscillate simultaneously in 

 the two stations. By another motion the sheets are drawn on in pro- 

 portion as the lines above mentioned are traced, so that when the 

 style s has passed over the entire surface of the plate of the manipu- 

 lator on which the message is written, the style s' will have gone over 

 in the same time a precisely equal surface of the chemical paper on 

 the plate of the receiving station. 



From the system of electric communication shown in the figure 

 these results follow : all the time the style s is on the metallic or 

 conducting part of the message sheet, the current from the battery is 



