APPLICATION OF THE ELECTPJC TELEGRAPH. 367 



bringing the star to the declination wires. When the star 

 is bisected, the observer strikes the key with his finger, the 

 circuit is formed, the electro-magnet brings the pen in 

 contact with the record plate ; and while in contact, the 

 plate descends in its ways, and a zero line is described on 

 the plate, from which all differences of declination are 

 afterward read. Professor Mitchell uses three declination 

 wires, and there are three zero lines obtained from the 

 standard star. When the difference of declination of one 

 star has been recorded, the plate moves upward about 

 the tenth of an inch, and is ready for the next record. 



To read the record of declination, the plate is laid on 

 a carriage and leveled by four screws : a movable arc, 

 divided into equal parts, whose values have been abso- 

 lutely determined and tabulated, is adjusted so that its 

 zero coincides with the zero line of the record. It then 

 glides on its ways, parallel to and just above the record 

 plate. A micrometer screw, and microscope, with a 

 spider's web, read the fractions of the equal parts into 

 which the arc is divided, with great facility and with 

 great accuracy. 



