TRANSIT OF VENUS, 1874. 259 



PAET II. 

 EXPEDITION TO EGYPT. 



SECTION I. OBSERVATIONS ON THE MOKATTAM HILLS, NEAR CAIRO. 



INTRODUCTION. 



Page 

 THE SITE, INSTRUMENTS, AND GEOGRAPHICAL DETERMINATIONS AT MOKATTAM : 



Arrival in Egypt ; Personnel ; Outline of Work 261 



The Site at Mokattam 261 



The Transit Instrument and Clock 263 



Value of a Revolution of the Transit Micrometer Screw. 263 



Equatorial Intervals of the Wires 263 



Errors of Level, Collimation, and Azimuth 263 and 264 



Reduction of the Observed Transits 264 



Errors and Rates of the Transit Clock 264 



Zenith Distances for Co-latitude 265 



LONGITDDE OF MOKATTAM KY GALVANIC SIGNALS : 



Preliminary Experiments 266 



Routine adopted 268 



Method of Signalling 268 



Errors and Rates of the Sidereal Standard Clock of the Royal Observatory, Greenwich, 



Table 1 269 



Errors of the Chronometers employed at Porthcurno on Greenwich Time, Tables II., III., 



IV., and V 270 to 273 



Errors of the Chronometer employed at Alexandria on Greenwich Mean Time, Table VI. 274 



Explanation of the Mahc-and- Break and Varley Methods of Signalling 276 



Adopted Errors of the Transit Clock at Mokattam 276 



Errors of the Alexandria Chronometer on Mokattam Mean Time, Tables VII. aud VIII. 277 



Longitude of Mokattam from each separate Group of Cable Signals, Table IX. . . . 278 

 Longitude of Mokattam separately deduced from the Make-and- Break aud from the 



Varley Signals, Table X. 281 



Collection of Results for Apparent Longitude, Table XI 283 



Collection of Resulls for Retardation of the Galvanic Current, Table XII 284 



Corrections for Personal Equation 284 



Final Longitude of Mokattam 288 



L L 



