for the year 1883. xvii 



by the time the electric current takes to traverse all the 

 cables and lines between the Australian cities and Green- 

 wich, and also by the personal equation of the observers. 

 The former quantity is obtained at once from the amount 

 the difference of local time differs when the signals coming 

 from Greenwich to Melbourne (from west to east) from those 

 going from Melbourne to Greenwich (from east to west). 

 For instance, if the difference between Greenwich and Mel- 

 bourne when signals are sent from Greenwich was two seconds 

 greater than when the signals were sent the reverse way, 

 it would show that half the amount — viz., one second — was 

 the time taken by the current to traverse the lines and the 

 cables and act on the signal instruments. This is called 

 retardation of current and relay time. When sent from 

 Greenwich, the signals being retarded, would arrive late, and 

 Melbourne, being to the east, would make the difference of 

 longitude too great. On the other hand, when the signals 

 are sent from Melbourne to Greenwich they go west, and the 

 retardation has the opposite effect, and would make the 

 difference of longitude too small. By taking the half of this 

 small difference we get the correction to be applied due to 

 the retardation. Unfortunately, however, it is practically 

 impossible to send direct from Greenwich to Melbourne, so 

 the operation has to be done in steps. The longitude of 

 Singapore from Greenwich has been determined in six steps by 

 different astronomers. First, from Greenwich to Mokattam, 

 in Egypt; second, Mokattam to Suez; third, Suez to Aden ; 

 Aden to Bombay, Bombay to Madras, and Madras to Singa- 

 pore. So that to connect Australia it was necessary to 

 exchange signals between Singapore and Australia. Were 

 it not for the great difficulty and danger to submarine cables 

 to connect them direct with land lines, the signals might 

 have been sent direct from Singapore to Melbourne ; but it 

 was necessary to establish an observatory at Port Darwin, 

 at the Australian end of the cable, as well as at Singapore. 

 In the British arrangements for observing the Transit of 



Venus last December the desirability of this undertaking was 



a 2 



