292 TRANSIT INSTRUMENT. 



Adjustment of great circle, or of a small circle, or their sines, are nearly coin- 

 the transit in- cident, we shall have very nearly as Z 3 (the zenith clistanee) 

 levelling' its ^^ ^ 3 (the polar distance), so is the value of 3 B, in degrees 

 axis, and ob- of the pole-star circle, to its value in degrees of a circle whose 



SeiS oAfme '"^^'"'^ ^' ^ ^' ^"^ ^' ^^^ ""^^i^^ '^ ^ '^ ^" ^ ^ very nearly as 

 between the 23 to 1, the error of the transit telescope, at the altitude 3 I, 



transit of the ^jn jjg measured by a scale {if it may be so called) 23 times as 



pole-star and of ■ ^c \ j > 



another more great as itself. 



distant from Now, let there be another star, A, whose northern meridian 



the pole ; the ,,.,,. n • • i ^ r i ^ 



R. A. bein<^ altitude is as small as it conveniently can be, for example, 5°:, 



given. whose polar distance is, therefore, 46° 30', and whose right as. 



cension is the same as that of the polar star; then, if the trar^sit 

 telescope be in the meridian, both these stars will pass through 

 it at the same time; but if it be out of the meridian by the quan- 

 tity IK, the star A will pass through it when it comes to C, 

 but the polar-star not till it comes to B, when the star A is got 

 to D, in its diurnal circle. 



The value of AC being therefore fo'4nd, by multiplying IK 

 by the cosine of its altitude AI, that value, being reduced to 

 the angular value to the radius PA, will give the tims of the 

 star A passing through the transit telescope, after the time 

 of its passing the meridian ; and the same operation being per- 

 formed for the pole-star as before directed, the difference of 

 these times will be the error in time of the transits, answering 

 to the given deviation IK of the tran-'it telescope. And tables 

 having been previously constructed for such stars as shall be 

 thought convenient, the transit telescope may, in a very short 

 space of time, be set to the meridian, with a degree of preci- 

 sion unattainable by any other method. 



If the star A precedes the pole-star in its passage under \)l\q 

 pole, no tables are requisite, nor any thing necessary to be 

 known but the exact difference of the right ascension between 

 the two stars ; for, having observed the transit of the star A 

 (the ii.:.Lrument being previously brought near the meridian, 

 suppose halfa degree), then eltvate the telescope to the pole- 

 star, by moving the horizontal adjustment of the axis: keep 

 the pole-star on the middle wire till the due interval of time 

 between their transits is elapsed; the instrument will then be 

 extremely near its true position ; and, by repeating the obser- 

 vation once more, will be brought to a perfect exactness. Or, 

 if another star, following the pole-star in its passage, be ob- 

 served 



