HISTORY AND PROGRESS. 163 



counteracting battery E 2 . But as the opposite magnetic effect 

 s 3 of the extra coils of relay n is only equalto that of s t , and 

 since s a is equal to the sum of s and S D it is evident that the 

 relay n will be acted upon by the difference of the magnetic 

 effects due to the line and the counteracting currents, or by 

 s, which is precisely the same as that produced when K alone 

 is depressed. The Morse M I will therefore also be set in motion. 

 Combinations have been made, also, by which in a single line, 

 at the same moment, two messages could be sent in one direc- 

 tion, whilst two were being received from the opposite direc- 

 tion ; that thus four independent communications could be 

 kept up. 



Other arrangements have also been made for telegraphing 

 in the same direction at the same time to different stations 

 along the line, both directly and by translation. 



Kramer, Bosscha, Maron, Edlund, and others have invented 

 also many similar and equally beautiful methods, all of which 

 have been tried, but none of which have found their way to 

 any extent to practical application ; and the reason is very 

 simply to be found in the varying resistance of telegraph 

 lines, and in the varying electro-motive forces of the 

 batteries, which occasion the inconvenience of having to 

 adjust the systems by means of resistances to compensate 

 these disturbances. Both these systems of telegraphing in 

 opposite directions, and of telegraphing in the same direction 

 more than one message at a time, must be looked upon as 

 little more than "feats of intellectual gymnastics" very 

 beautiful in their way, but quite useless in a practical point 

 of view. 



96. Automatic Printing Telegraph. Professor Wheat- 

 stone, to whose inexhaustible fund of invention this modifi- 

 cation owes its being, described it in a paper read before the 

 Paris Academy in January, 1859. 



It consists principally in the mechanical transmission of 

 signals by means of contacts given by series of perforations 

 in bands of paper previously prepared and drawn through 

 the manipulator ; the signals being printed by the recording 

 instrument. 



M2 



