I''- 



HISTORY AND PROGRESS. 177 



imparts motion by the friction of a small roller pressing upon 

 its under surface. 



When no current is moving in the line, the style rubs on 

 the surface of the paper without producing any mark ; but 

 as soon as a current in the right direction is established, 

 the salt solution with which the paper is saturated becomes 

 decomposed, and leaves a blue mark upon the surface. 



When the circuit of the current is made and broken 

 repeatedly, a series of dots or dashes is imprinted upon 

 the receiving disc, the lengths and succession of which 

 depend upon the manipulation of the key or contact 

 maker. 



To render these dots and dashes intelligible, Bain has 

 adopted an alphabetic code like that of Morse. 



The paper is rendered sensitive by being saturated in a 

 mixture of prussiate of potash dissolved in water, to which 

 are added two parts of nitric-acid and two of ammonia. 



An interesting rather than practical modification of this 

 apparatus consists in substituting a revolving disc with style 

 travelling in a spiral curve, exactly similar in form to that 

 just described, at the sending station, for the key; only, 

 instead of being covered with prepared paper, it is left naked, 

 and letters or words written upon its surface, within the 

 limits of the journey of the style, in some insulating material ; 

 so that, when the style passes over the insulating writing, 

 the current is interrupted. 



When both the discs are made to revolve synchronously, 

 which is very difficult, it is evident that the paper at the 

 receiving station will be marked with a dark spiral curve 

 broken in just those places where the letter, written down on 

 the transmitting dial, interrupts the passage of the current ; 

 and a facsimile of the writing will be obtained. 



The same idea has been carried out, in a more convenient 

 form, by the employment of short cylinders instead of the 

 discs, on the sides of each of which are printers or styles 

 running to and fro upon long screws as the cylinders are 

 turned round. 



But the apparatus by which Bain has earned most credit, 



