HISTORY AND PROGRESS. 43 



an escapement worked by an electro-magnet, and probably, 

 in its turn, suggested many of the subsequent inventions. 



With three batteries at the sending station, Davy was 

 enabled, by reversing the currents, to deflect at pleasure the 

 tongues or needles of six relays at the receiving station. 

 This was done by putting in each line two relays, similar in 

 construction to those used by Wheatstone in conjunction 

 with his alarm, one being acted upon by positive currents 

 and the other by negative only. Beyond the second relay 

 coils, the three lines were connected together with the fourth 

 line wire, which was used as a common return circuit. 



The receiving apparatus consisted of a sheet of cloth or 

 other chemically-prepared material, drawn between a metallic 

 cylinder and a series of six platinum rings, placed equi- 

 distant on the outside of a wooden drum. Each of these 

 rings was connected with the contact points of one of the 

 relays, and a common local battery was inserted between all 

 the tongues or needles of the relays and the metallic cylinder, 

 so that when the needle of either of the relays was deflected, 

 the current of the local battery passed through the chemi- 

 cally-prepared cloth to the metallic cylinder, producing 

 a dot. 



On the arrival of a current the metallic cylinder was moved 

 forward a certain distance by means of the clockwork. 



The operation of successively opening and closing the 

 circuit at the sending station, imparted to the cylinder at 

 the receiving station a rotatory motion resembling that of 

 the long hand of a clock governed by the pendulum and 

 escapement. 



The cloth used for receiving the marks was impreg- 

 nated with iodide of potassium and muriate of lime. Six 

 longitudinal lines, intersected by transverse ones at similar 

 distances, divided the whole surface of the cloth into regular 

 squares, which facilitated reading off messages. 



44. The system which we have next to notice is another 

 invention of our ingenious countryman, Professor Wheat- 

 stone ; this is his first dial instrument, patented in 

 1840. The apparatus in question seems to have undergone 



