809 



TRANSIT, OR TRANSIT INSTRUMENT. 



TRANSIT, OR TRANSIT INSTRUMENT. 



310 



altitude and the slow-motion screw are seen at b. There is a caution 

 to be given here. The tail-piece should never be tightly nipped, unless 

 the instrument is used for observing declinations, and it and the 

 tangent screw must be released when the observer uses the azimuth 



screw for bisecting any object, such as a mark or a star at a given 

 moment. In large transits there are generally two small circles fixed 

 on each side of the transit towards the eye-end. They are here more 

 convenient for setting, and it is easy to pass rapidly from one star to 







Trnnnit Inrtrnment. 



another, when both the circles are previously set. There ia great 



ity in the graduation of the setting circles. In large instruments 



which are used for Home time in the same position, it is best to make 



read polar distance or declination, ru-adj listing the circles 



whenever the transit a reversed. With a portable transit, which ia or 



ought to be very frequently reversed, a graduation to altitudes one way, 

 which becomes zenith distances when reversed, is perhaps as convenient 

 as any, though a slight computation for each star is required to form 

 working catalogue. The telescope in this instrument is not inserted in 

 the ordinary manner. The central portion, from c to d, is in one tube, 



