XXI. On the Determination of the Longitude of the Observatory of Cambridge by 

 Galvanic Signals. By the Rev. J. Challis, M.A., F.R.S., F.R.A.S., Plumian 

 ■ Professor of Astronomy and Experimental Philosophy in the University of 

 Cambridge. 



[Read May 15, 1854] 



On March 10, 1853, I received a proposal from the Astronomer Royal to make an 

 exchange of galvanic signals between Greenwich and Cambridge, for the determination of the 

 Longitude of the Cambridge Observatory, to which proposal I immediately signified my 

 assent. The Greenwich Galvanic Signal Apparatus had already been tested by giving and 

 receiving signals between Greenwich and Dover, but had not yet been applied to the deter- 

 mination of Longitude. Various arrangements were to be made, which necessarily occupied 

 time. In the first place, I had to apply to the Electric Telegraph Company for permission 

 to make use of the telegraphic lines and apparatus on the Eastern Counties' Railway. I am 

 bound to say that my application was responded to by the Directors in the most liberal 

 manner, and I was immediately put in communication with the Company's Engineer, Mr Edwin 

 Clark, for the purpose of facilitating the requisite arrangements. The completion of the cir- 

 cuit from Greenwich to Lothbury in the Strand, London, having been undertaken at Mr Airy's 

 request by Mr Walker, Engineer and Superintendent of Telegraphs of the South-Eastern 

 Railway, Mr Clark provided carefully for the junctions at Lothbury and the Eastern Counties'" 

 Terminus, and for the exclusive use of a line for the transmission of signals to and from the 

 Cambridge Station. I was also furnished with a galvanic battery, a separate signal-needle, 

 and all the personal aid that I required. I did not propose the carrying a temporary wire 

 from the Cambridge Station to the Observatory, (a distance of about 2i miles), thinking 

 that great expense and trouble would be thereby incurred, but had I considered this 

 arrangement necessary for the success of the experiment, I have reason to say that the 

 authorities of the Electric Telegraph Company would have lent their aid in effecting it. 



All necessary preparations having been made, the exchange of signals eventually took 

 place on the nights of May 17 and 18. I cannot better commence my account of the 

 operation than by citing the scheme for conducting it communicated to me by Mr Airy, 

 which was closely followed. It should first be premised that this method of determining 

 longitude is in principle the ordinary method of signals, the instant of the same signal being 

 recorded in the mean or sidereal times of two observatories, so as to give by the difference 

 of times the difference of the longitudes of the observatories. On the supposition of the 

 instantaneous transmission of galvanic action, two needles, however distant from each other, 

 would start at the same instant by the completion of a galvanic circuit, and afford the 

 means of recording the times at two positions of the same event. And if the galvanic 

 Vol. IX. Part IV. 63 



