HENKY AND THE TELEGRAPH. 323 



Among examples of " magnetic " telegraphs which might properly here 

 receive a passing notice, are the four following : 



1837. The so-called " mechanical " or chronometric telegraph of Mr. 

 William F. Cooke, of London, comprising two synchronously revolving 

 cylinders (or escapements) at the two stations, arrested simultaneously 

 by a magnetic armature detent, somewhat after the general principle of 

 Eonalds's synchronous dials of 1810, previously mentioned. This form 

 of dial telegraph was worked by Mv. Cooke in April, 1837.* 



1837. The first letter-printing telegraph, devised by Mr. Alfred Vail, 

 of New Jersey, in September, 1837, comprising a imnting- wheel pro- 

 \ided Avith spring tyi)e for the letters of the alphabet, projecting radi- 

 ally from its periphery, and corresponding with the teeth of an escape- 

 ment wheel on the same shaft or axis, driven by ordinary clock-work, and 

 regulated by a pendulum. The pendulum oscillating as a free armature 

 between two electro-magnets, was arrested by one of the magnets when 

 the desired letter was reached, and another electro-magnet, with lever 

 armature, simultaneously drew down the spring type of the letter- wheel 

 upon the fillet of paper beneath it.t This ingenious arrangement like 

 the dial telegraph of Konalds, and that of Cooke (independently con- 

 trived but a short time previously), required a synchronous movement 

 of the clocks and their pendulums at the two stations. Eighteen years 

 later, a printing telegraph on the same principle was very successfully 

 worked out and operated by Mr. David E. Hughes, of Kentucky. 



1838. The electro-magnetic chemical telegraph of Mr. Edward Davy, 

 of London, comprising a chemically marking or recording cylinder, op- 

 erated by a clock-work escapement and the armature of an electro-mag- 

 net. Eelays of circuits were also included, operated by a galvanometer 

 needle.f 



1839. The dial telegraph of Prof. Charles Wheatstone, of London, 

 completed by him in November, 1839, comprising an escapement and 

 index operated by the step-motion of an electro- magnetic armature. In 

 this arrangement, the synchronous motions and indicating positions 

 on the terminal dials were effected entirely by the specific number of 

 galvanic impulses given to the transmitting and receiving escapements.§ 

 The principle of this transmission was in 1846, skillfully and success- 

 fully applied by Mr. Royal E. House, of Vermont, to a " printing tele- 

 graph." 



* "Mr. Cooke's Case," before the arbitrators. The Electric Telegraph, etc. by W. F. 

 Cooke; part ii, p. 2'6. It appears that this arrantijement was devised by Mr. Cooke in 

 1836. 



t The American Electro-magnetic Telegraph, by Alfred Vail, 1845, pp. 159-171. 



tTho English patent of Edward Davy, July 4, 1838, No. 7719. 



$ "Professor Wheatstone's Case," before the arbitrators, in 1840, p. 101. 



