416 THE POPULAR SCIENCE MONTHLY. 



catches in a notch corresponding to the blank on the type-wheel. The 

 adjustment can also be made by hand. 



Lastly, the shaft (I) carries a third cam, which, at each revolution 

 of this axis, engages with a very coarse-toothed wheel (T'), set on the 

 same axis as the type-wheel, and pushes it a little forward or backward 

 without detaching it from the driving-gear. Small discrepancies' be- 

 tween the velocities of the type-wheel and chariot are thus corrected 

 as often as a letter is printed. This contrivance serves to keep the re- 

 ceiving instrument from gaining or losing on the sending instrument 

 during the transmission of a message. The type-wheel of the receiv- 

 ing instrument must be adjusted before the message begins, so as to 

 make the two instruments start at the same letter. 



Suppose a metallic cylinder, permanently connected with the earth, 

 to be revolving, carrying with it on its surface a strip of paper freshly 

 impregnated with cyanide of potassium. Also suppose a very light 

 steel point permanently connected with the line-wire, and resting in 

 contact with the paper. Every time that a current arrives by the 

 line-wire, chemical action will take place at the point of contact, and 

 the paper at this point will be discolored by the formation of Prussian 

 blue. This is the principle of Bain's electro-chemical telegraph, which 

 leaves a record in the shape of dots and dashes of Prussian blue. The 

 apparatus for sending signals is the same as in Morse's system. The 

 paper must not be too wet, or the record will be blurred ; neither must 

 it be too dry, for then no record will be obtained. 



An autographic telegraph is one which produces at the receiving 

 station a fac-simile of the original dispatch. The best known-instru- 

 ments of this class are those of Bonelli and Caselli. We shall describe 

 the latter. 



Fig. 16. 



Principle of Caselli's Telegraph 



At the sending station a sheet of metallized paper, with the dis- 

 patch written upon it in a greasy kind of ink, is laid upon a cylindric 

 surface (M, Fig. 16). At the receiving station there is a similar cylin- 

 dric surface (R), on which a sheet of Bain's chemical paper is laid. 

 Two styles, driven by pendulums which oscillate with exact syn- 

 chronism, move over the surfaces of the two sheets, describing upon 



