ON THE SQUARE BAR MICROMETER. 169 



9. Onenfation of the square to tlic real diurnal motion. — Suppose the telescope to 

 be equatoriallj mounted, provided with a micrometer position circle, and in good 

 adjustment, so that the instrumental pole is very close to the true pole, say 

 within a minute of arc, as it should be in a well mounted instrument. Turn 

 to a star on or very near the meridian, and set the square by the position 

 circle so that the star traverses the diagonal, or parallel to it, as nearly as 

 possible. Call the reading of the position circle P, and observe the times of 

 transit as desci'ibed in article 12. Since on the meridian A^^ disappears, ^j = p', 

 and is found by equation (32), The reading P — j>; will be the true zero of 

 position at which the position circle should be thereafter set to orient the 

 square to the true diurnal motion. If the equatorial is in good adjustment it 

 will answer for all parts of the sky. If not, the correction due to the equatorial 

 errors may be determined as shown in the text books (Chauvenet, II., p. 261). 



If it be inconvenient to make this adjustment on the meridian, an extra- 

 meridian determination can be made, the correction for refraction, A^j, being 

 found by (22) and p by (32). Then the reading for the setting to true diurnal 

 motion will be P — (/'+ A^;). 



It is scarcely necessary to say that the adapting tube and micrometer should 

 be so mai'ked, or fitted ,to each other, that the latter may alwaj's be inserted 

 at a determinate position angle in the telescope. 



If the telescope be not provided with a position circle the adjustment can 

 still be made nearly as above described, with a little additional trouble, by 

 several approximations, the tubes being carefully marked when the true position 

 zero is found. 



The refraction corrections to be used in reducing observations made with 

 the square adjusted by this article are (18) and (20), or (21). 



10. Orientation of the square to the apparent diurnal motion. — The adjustment by 

 the previous article is to be preferred, on the score of convenience, when the 

 micrometer is to be actively employed, as it need be made once only for all 

 observations. Where, however, the telescope is not mounted equatorially, or the 

 equatorial errors are unknown, or for other reasons, it may be necessary to 

 adjust the square on the occasion of each observation. The orientation will 

 then be more conveniently made with respect to the apparent diurnal motion. 

 Several cases may arise. 



Case I. Equatorial provided with position micrometer circle, instrumental ad- 

 justments unknown. Point the telescope as nearly as possible to the hour angle 

 VOL. XI. 22 



