276 



TRANSIT OF VENUS, 1874. EGYPT. 



another group ; these groups are indicated in the table above by the words 

 " make " and " break " respectively. 



In what is called Varley's method the circuit was completed at the 

 sending station for 50 seconds, and broken for 10 seconds, using alternately 

 the positive and negative key. The signals corresponding to break of circuit 

 only were made use of. At the receiving station* the extent of motion of 

 the beam of light, from zero on the scale, having been first noted, a mark 

 was made at the middle point of its excursion. Just before the expected 

 break, the circuit at the sending station will have been closed for nearly 

 50 seconds, and the beam of light at the receiving station will have come to 

 rest. At this moment the observer shifts the position of the scale so as to 

 bring the beam to the zero point, and, on break of the circuit, notes the time 

 at which the beam transits the provisional mark, which is the phase of signal 

 observed. The groups are formed without any distinction as to quality of 

 current, positive or negative. 



The times not fractional indicate means of times of contacts at the sending 

 station, one or two times being occasionally omitted to make the mean 

 identical with the time of some one particular contact, or with a time half 

 way between those of two successive contacts. The fractional times are the 

 means of the observed times of the corresponding signals at the receiving 

 station, noted as described above. Two times are by chance not fractional, 

 4P'00 in Group 47, and 47 S- 00 in Group 71 ; they however indicate, as a 

 little consideration would show, means of times of signals received. 



We have, in the preceding table, the error of Hewitt 890 (at Alexandria) 

 on Greenwich Mean Solar Time ; it is now necessary to find the error of the 

 same chronometer on Mokattam Mean Solar Time. 



From the numbers given in Table XVII. (page 314), Errors and Rates of 

 the Mokattam Sidereal Clock, Dent 1914, were adopted as follows : 



These errors and rates are determined entirely from the observations of 



* As before mentioned a condenser was placed in the circuit, between the line and the recording 

 instrument, to improve the character of the signal. 



