CHAP, i.] MIRRORS AND REFLECTING INSTRUMENTS. 217 



is connected a rod normal to its surface, which slides in a ring fixed 

 to a fork, the axis of which bisects the direction of the incident and 

 reflected rays of the heavenly body observed. This fork is at its other 

 end connected with an axis which is parallel to the axis of the 

 earth, and which is made to rotate with a movement uniform with 

 that of the earth itself. 



A graduated circle determines the direction of the axis of the 



FIG. 153. The siderustat. 



fork, so that the angle which it makes with the axis of the earth varies 

 according to the polar distance of the star. The hour angle of this 

 latter being given the moment when the observer wishes to begin, the 

 instrument is moved in such a manner that the rays of the star lie in 

 the plane which passes through the star and the axis of the telescope, in 

 which they are retained during the time of observation by the clock- 



