290 TRANSIT OF VENUS, 1874. EGYPT. 



The wind continued in the S.W., and as the day began to dawn it was 

 evident that the sky was clearing from that quarter. 



I set the two observing clocks, Dent 2009 and 2015, as nearly as possible 

 to true sidereal time, and then compared them with the transit-clock. 



The " Lee Equatorial," with which I observed the Egress of Venus, was the 

 famous instrument by TULLEY, used by the late Admiral SMYTH, and fully 

 described by him in the first column of his Cycle of Celestial Objects. It was 

 purchased by the Government expressly for the Transit of Venus. At 

 Mokattam the polar axis was supported by massive structures of timber, 

 which rested upon the solid rock, but otherwise the instrument, accessories, 

 mounting, and driving-clock were in every respect the same as formerly, but 

 perhaps, rather the worse for wear. The hour-circle, which was also the 

 driving-circle, had been seriously distorted by a fall, and could not be 

 perfectly restored, but it answered its purpose well. The instrument was 

 protected by a large portable wooden hut constructed at Greenwich. 



The tube of the telescope had been shortened a little to admit of the use of 

 a new solar diagonal reflector. Slow motion in Right Ascension to a limited 

 extent was effected by the slipping piece, described by Admiral SMYTH, which 

 carried the eye-piece at times out of the true centering. For the proper 

 equalisation of the illumination of the images of the double-image micro- 

 meter eye-piece it was imperative to have the latter fairly centered, which 

 was effected by trial, an assistant manipulating the driving-clock. 



The double-image micrometer and solar reflector are represented in 

 Plate III. The power used was 158. The internal contact was observed 

 with a negative eye-piece of 212,* constructed to correct atmospheric 

 dispersion by tilting the eye-lens, but the definition being very good the 

 eye-lens was not disturbed. 



The Sun rose behind a bank of cloud ; indeed, the sky was quite thick 

 towards the north and east horizon, but clear from the south-west up to the 

 zenith, and the clouds were breaking still further in the desired direction. 

 As the Sun rose higher the clouds became more broken. The intervals of 

 obscurity, however, were long and trying, especially the last, which began 

 eight minutes before internal contact, and lasted about five minutes. 



As opportunity occured, I obtained the following measurements with the 

 micrometer for the diameter of the planet, and the distance between its 

 approaching limb and the limb of the Sun. Sometimes the images were 

 distorted by atmospheric tremor ; at others the definition was excellent. I 



* Not 150, as stated in the Parliamentary Report, page 16. 



