MERIDIONAL OBSERVATIONS AT MOKATTAM. 263 



The Transit instrument, with its fittings, mounting, and stone piers, and the 

 Transit Cloclc, with its solid tripod stand, were in every respect exactly similar 

 to the Honolulu instruments already described. The wooden observatory, 

 13 feet square, was also similar. 



The great stone upon which stood the vertical piers was bedded in Port- 

 land cement laid on the surface of the solid rock. Observations for local 

 time were commenced October 24. 



The stars observed for clock-error were taken generally from the list 

 employed at the Royal Observatory, Greenwich, the Right Ascensions being 

 brought up from the Greenwich Catalogue of 2760 Stars for the Epoch 1864. 

 For the determination of azimuth-error the stars a, 5, x Ursee Minoris, or 

 Cephei (Hev.) 51, were employed. 



The valm of on,e revolution of the transit-micrometer-screw (which carried all 

 the wires of the reticule) was found by numerous observations of Polaris to 

 be 56"'43. The integer revolutions of the screw were numbered in the 

 observing-books so as to increase towards the screw-head ; the optic axis 

 coincided approximately with the center wire when the reading was 19 r- 7 

 (see Table XIV.). The position of the transit axis is always denoted by the 

 record of the micrometer-screw-head being on the East or West side of the 

 telescope. 



The system of wires remained perfect during the observations, and the 

 equatorial intervals of the five vertical wires from the center wire were found 

 from the transits of 150 time-stars as follows : 



8 



Wire I. (nearest to the screw-head) 28-610 



II 14-143 



-. Ill o ooo 



IV H'igfi 



V. (farthest from the screw-head) . 28'58i 



The imperfect transits are very few in number, and all the observations of 

 the polar stars were made at the center wire. 



The mean of the five wires has been considered as sensibly coincident with 

 the center wire. 



The Error of Level (Table XIII.) was found with the hanging spirit-level. 

 The value of the graduations engraved on the glass bubble was tested by the 

 makers before the Expedition left England, when it was found that 

 50 divisions were equivalent to one minute of arc, which value has been 

 always used. 



