94 Telegraphic Determination of the Difference of 



Art. XII. — Account of the Telegraphic Determination of 

 the Difference of Longitude between Melbourne ancl 

 Hobart Town in the Year 1875. 



By E. J. White, Esq. 



[Read before the Royal Society of Victora, 14th December, 1876.] 



The late transit of Venus having been successfully observed 

 at Hobart Town by the American party under the command 

 of Professor Harkness, it became a matter of necessity to 

 obtain the longitude of the observing station. Instead of 

 an absolute determination with reference to the meridian of 

 Greenwich, which would have required months, or even 

 years, for its successful execution. Professor Harkness re- 

 solved to obtain it differentially from Melbourne, the two 

 places being connected by means of the land lines and 

 submarine cable of the electric telegraph ; and for the pur- 

 pose of arranging a scheme for carrying out this intention he 

 visited Melbourne towards the latter end of November, 1874. 

 Having settled upon a plan of operation with Mr. EUery, 

 and having obtained the consent and promise of hearty 

 co-operation of Mr. Warren, the managing engineer of the 

 Tasmanian Cable Company, and Messrs. James and Payter, 

 the Melbourne managers of the electric telegraph, he re- 

 turned to Tasmania, and immediately after he had observed 

 the transit of Venus a few unsuccessful attempts were made 

 to send the signals direct, with automatic repeaters, between 

 Melbourne and Hobart Town. Soon after this. Professor 

 Harkness had to accompany the " Swatara " during her 

 cruise in the South Pacific, to collect the different parties 

 of American observers in that part of the world, and further 

 attempts were deferred till his return. Advantage was 

 taken of the interval to improve the repeating apparatus, 

 and on his return at the end of January the signals were 

 transmitted without any difficulty. 



At Hobart Town the observations were taken by Pro- 

 fessor Harkness, who employed a portable transit instrument 

 of 2J inches clear aperture and 80 inches focal length, with 

 a magnifying power of 60 diameters. The transit was 

 reversed each night near the middle of the observations. 

 Three clock stars and two azimuth stars were observed in 

 each position of the axis, and from the complete set of ten 



