490 PROFESSOR CHALLIS, ON THE DETERMINATION OF THE LONGITUDE 



greatest care was taken by all parties engaged in the Telegraph Office to avoid making noises 

 that might disturb the experiment, but some inconvenience was unavoidably experienced from 

 the occasional passage of railway carriages. 



On May 18, the operators and the distribution of their duties, both at Greenwich and 

 Cambridge, were the same as on the preceding night, excepting that Mr Dunkin and Mr C. 

 Todd changed places. My objects in having the signals observed simultaneously by two 

 observers, were, to check mistakes in writing down the seconds, and to have the opportunity 

 of deducing difference of personal equations in a mode of observing in which the fraction of a 

 second is estimated in a different manner from that in which it is estimated in a transit 

 observation. In order that the kind of observation might not be entirely strange to the 

 observers, both Mr Breen and Mr Todd practised, a short time previous, taking notes of 

 time of sudden jerks which I gave at arbitrary intervals to a temporary substitute for the 

 galvanic needle. 



The Cambridge Observatory time was transferred to the Telegraph Station by means of 

 three chronometers, beating half-seconds, one going mean solar time, and the other two 

 sidereal time*. The two sidereal chronometers were compared with the transit-clock by the 

 intervention of the solar chronometer just before we left the Observatory for the Station, they 

 were compared with the solar chronometer at the Station both before the signals commenced 

 and when they were finished, and lastly they were compared again with the transit-clock 

 by means of the solar chronometer on our return to the Observatory. All the comparisons 

 were made by coincidences of beats. The signal-times were noted by one of the sidereal 

 chronometers. 



Exactly at ll h Greenwich Mean Time of May 17, the first signal reached the Cambridge 

 Station. Four others followed at intervals of about two seconds, and then five signals were 

 similarly sent from Cambridge. These by previous understanding signified that all was 

 right. The batches of signals then commenced, and were continued to 12 h , Greenwich giving 

 signals during the first and third quarters of the hour, and Cambridge during the second and 

 fourth. The following is a list of the batches of signal-times recorded at Greenwich and 

 and Cambridge on May 17, inclusive of those recorded by Mr Breen : — 



* A conveyance was hired for the transport of the chrono- 

 meters and the party of observers, each chronometer was held 

 by hand, and when it was necessary to pass over the street- 



pavement, we alighted from the conveyance and carried the 

 chronometers walking. 



