302 TRANSIT OF VENUS, 1874. EGYPT. 



between 1875, May 21 and June 2, some 60 comparisons were made of the 

 relative values of revolutions of the two screws, using different parts of the 

 screws, all showing a very close agreement. The half and quarter revolutions 

 were also tested. It was found that 4'320 revolutions of the screw of the 

 double-image micrometer were equivalent to one revolution of the screw of 

 the transit-micrometer. The value of the latter had been very carefully 

 determined at Mokattam to be 56"'43 ; re-determined at Greenwich, 56"-35 

 was obtained. Adopting 56"-40, the value of one revolution of the double- 

 image screw is 13"'06. 



The measured semidiameters of Venus in transit were 



2^-419 and 2 reT '488, 



corresponding to 



3i"-5 9 and 32"'5o. 



The mean of the two gives 



8 r = + -o"62, 



which, although not very trustworthy, agrees very closely with the value 

 obtained with telescopes of nearly the same aperture at Honolulu and Roorkee. 



Time of Internal Contact at Molcattam inferred from the Cusp-Measures. 



Taking 8 r = + 0"'62, as above, the Greenwich Sidereal time of internal 

 contact inferred from the first five cusp-measures is 



h m a 



ii. 17. 9-2, weight 9, 

 11.16.57-8, 7, 

 11.17.14-5, 6, 

 ii. 17. 7-1, 5, 

 11. 1 6. 56-2, 4. 



The mean with the weights assigned is 



5 s -6, 



which is from 10 to 12 seconds earlier than the telescopic contacts, proving 

 that the cusp-measure is too great, as was found at Honolulu and Roorkee. 



G. L. T. 



