The hour circle is also read by microscopes, and the instrument can be used in both positions (tube preceding and following) for elimination of the effect of flexure on the position angles.
It is equal to the angle at the pole between the hour circle through the body and the meridian, but is usually expressed in time.
I i shows the whole instrument on a small scale with the telescope directed to the pole, and the hour circle set 6" from the meridian.
The hour circle has two toothed circles cut upon it, one acted upon by a worm screw mounted on the pier and driven by clockwork, the other by a second worm screw attached to the polar axis, which can be turned by a handle in the observer's hand and thus a slow movement can be given to the telescope in right ascension inde FIG.
But when much larger instruments are required the hour circle becomes inaccessible from the floor, and means have to be devised for reading both circles from the eyeend.
