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a complete catalogue of the exact positions of the stars, but 

 also a perfect map of the heavens. 



In case we do not read our Declinometer scale, we can 

 determine the declination from the chart, within one-tenth 

 of a minute of arc. The precision with which this machine 

 will map stars is all that could be desired ; since if two 

 charts of the same zone, made on different nights, be 

 placed one over the other, the stars will be superimposed 

 so that the eye can detect no difference. 



By means of movable adjustments, we can set the ma- 

 chine (having our sheet ruled for Right Ascension and 

 Declination), so that it will give the position of the zone, at 

 the beginning of the year, without sensible error. For ad- 

 justing in Right Ascension, the cylinder can be moved 

 about its axis, being held in position by a friction block. 

 >For the declination, we lengthen or shorten the rod (/), by 

 means of the screw (x), fig. 2. The scale for declination 

 can be varied at pleasure, by changing the position of the 

 connecting rod (,/), on the lever /, fig. 1. This apparatus 

 can be adapted to any Telescope either transit or equatorial ; 

 neither does its use interfere with the ordinary work for 

 exact positions. 



In the observation of Asteroids on the meridian, a great 

 deal of time is wasted, especially when the error of the 

 Ephemeris is considerable. And even when the error is 

 only 2' or 3' in declination, in certain portions of the 

 heavens, it is almost impossible to find the body with a me- 

 ridian instrument. We believe it is a rule adopted by 

 Prof. Airy, the Astronomer Royal, to observe only those 

 Asteroids whose places are pretty accurately known. 



This apparatus affords great facility in finding these 

 bodies, when we have an approximate Ephemeris ; since 

 it is only necessary to observe, on two nights, a short zone 

 of five minutes in Right Ascension and 10 minutes in De- 

 clination. The comparison of these two charts will at once 

 show which is the planet, provided it is included within 



[Trans, iv] 24 



