LONGITUDE OF MOKATTAM. 285 



determined clock-error. After the return of Captain Browne from Egypt 

 the necessary comparisons for determination of these various differences 

 among the observers who took part in the longitude work were made. 



To ascertain the personal differences in giving signals, the Greenwich 

 Chronograph was employed. Any two observers to be compared made 

 contacts alternately with a key such as was used in the actual longitude 

 work. The corresponding signals were registered on the Chronograph, the 

 times of contact being so symmetrically arranged that the means of all the 

 times referred for each observer to the same moment of absolute time. 

 Several sets of observations were made, the observers being interchanged in 

 various ways. 



For the personal differences in receiving signals, a galvanometer, such as 

 was used in the longitude work, being mounted in a convenient position, an 

 assistant, placed in a different apartment, gave signals, which the various 

 observers noted, using the same clock. Several different sets of observations 

 were made. 



A discussion of the whole of the comparisons made gave the following 

 results, the initials E, C, B, and H referring respectively to Mr. Ellis, 

 Mr. Criswick, Captain Browne, and Mr. Hunter. C is taken as the standard 

 of reference : 



Resulting difference 

 In giving signals. In receiving signals. 



B S 



C E, + 0-068 o'ogo 



C B, + o'o33 o' io5 



C H, + o'o38 0-043 



Thus E makes contact O s> 068 earlier than C, but notes a signal O s> 090 later 

 than C. These numbers refer to the giving and receiving of signals on the 

 land lines, and to giving on the submarine line, but not to receiving on the 

 submarine line. 



In receiving longitude-signals on the cable the comparatively slow motion 

 of a beam of light was observed in two different ways. In one the first start 

 of the beam was observed, in the other the time of transit of the beam over a 

 provisional mark was observed. Some trials of the latter method made in the 

 month of August 1874, in the first instance with Mr. C. F. Varley's artificial 

 cable (kindly lent by him for experiment), and afterwards at Forth curno on 



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